VBA, 512  489 479 byte
Sub f(A,B):Debug.?">>>>>->>>>->--<<<"&Replace(Replace(Replace(A,"<","<<<[+]-<<"),">",">>>>>>>[+]-<<"),".",">>-<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>>+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]-<<")&">>[>>>>>]<<<<<[+<<[-]<<<]>++[-->>>>>++]+[-<<<<<+]->>"&Replace(Replace(Replace(B,"<","<<<<<"),">",">>>>>"),",","[-]>>->[>>>>>]+[-<<<<<+]->>>[-<<<[<<<<<]>>>>+[->>>>>+]-<<+>>>[>>>>>]+[-<<<<<+]->>>]>>-[<<<<<]>>>>+[->>>>>+]<<"):End Sub
Giải trình
Mã VBA thay đổi mã brainfuck theo cách, rằng đầu ra của đoạn A sẽ được lưu trong danh sách và đầu vào của đoạn B sẽ được đọc từ danh sách đó.
Đầu tiên nó khởi tạo một số biến 
>>>>>->>>>->--<<<
Sau đó nó đọc đoạn A và thay thế tất cả <bằng cách <<<[+]-<<, mỗi >bằng >>>>>>>[+]-<<và mọi .bởi thói quen lưu trữ
>>>>>>>[+]-<<"),".",">>->+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]--<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>+<<
sau đó nó xóa bộ nhớ của đoạn A và thay đổi danh sách được lưu trữ, vì vậy nó có thể được đọc làm đầu vào cho đoạn B:
>>[>>>>>]<<<<<[+<<[-]<<<]>++[-->>>>>++]+[-<<<<<+]->>>>>-<<<
Sau đó, đoạn B sẽ được đọc, mọi thứ <sẽ được thay thế <<<<<, mọi thứ >sẽ được thay thế >>>>>và mọi thứ ,sẽ được thay thế bởi thói quen đọc danh sách:
[-]>>->[>>>>>]+[-<<<<<+]->>>[-<<<[<<<<<]>>>>+[->>>>>+]-<<+>>>[>>>>>]+[-<<<<<+]->>>]>>-[<<<<<]>>>>+[->>>>>+]<<
Mã nguồn Brainfuck
Đây là nguồn của tôi cho các phần brainfuck của mã. Tôi sẽ giải thích chi tiết sau.
[
Tape: tempMem data out/in dataAnchors outAnchors (all repeating)
dataAnchors of Snippet A: -1 = memory has been used, -2 = active memory cell for copying to out/in
dataAnchors of Snippet B: -1 = active memory cell for copying from out/in
outAnchors of Snippet A: -1 = start of list, -2 = next output position
outAnchors of Snippet B: -1 = character has been read (or start of input)
]
### Init
>>              two blank data cells (for non wrapping pointer)
>>>-            set start of input
>> >>-          set first "used" flag
>--             set end of input
<<<             return to first usable data cell
### A move right routine
>>>>>           move to the next data cell
>>[+]-          clear and set "used" flag
<<              return to data cell
### A move left routine
<<<[+]-         clear and set "used" flag of previous data cell
<<              go to data cell
### A print routine
>>-             set flag
<<[             while value greater 0
  -               decrement value
  <+              increment tempMem
  >>>>+[-<<<<<+]  find start of input
  +[-->>>>>++]--  find end of input
  <<+             increment input
  >>+[-<<<<<+]-   find start of input
  <++[-->>>>>++]--find flag
  <<              go to active data cell
]
<[->+<]         move stored value back to data
>>>>+[-<<<<<+]  find start of input
+[-->>>>>++]    find end of input
>>>>>-          set new end of input
[-<<<<<+]-      return to start of input
<++[-->>>>>++]- return to and delete data flag
<<              go to data cell
### After snippet A: Delete memory of A and configure out/in list
>>[>>>>>]<<<<< go to last used data
[+<<[-]<<<]    delete each data
>++[-->>>>>++] find and delete end of input flag
+[-<<<<<+]-    go to start of input
>>             go to first data cell
### B move right routine
>>>>>          go to next data cell
### B move left routine
<<<<<          go to previous data cell
### B reading routine
[-]                      set cell = 0
>>-                      set flag
>[>>>>>]+[-<<<<<+]-      find start of input
>>>[                     if value greater 0
  -                        decrement value
  <<<[<<<<<]>>>>           go to initial start of input (combined with next)
  +[->>>>>+]-              find mem cell flag
  <<+                      increment mem cell
  >>>[>>>>>]+[-<<<<<+]->>> return to input cell
]
>>-                      set new start of input
[<<<<<]>>>>              go to initial start of input (combined with next)
+[->>>>>+]               find and delete mem cell flag
<<                       go to mem cell
Đầu ra cho trường hợp thử nghiệm 1: f ",[..,]",",[...,]"
>>>>>->>>>->--<<<,[>>->+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]--<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>+<<>>->+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]--<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>+<<,]>>[>>>>>]<<<<<[+<<[-]<<<]>++[-->>>>>++]+[-<<<<<+]->>>>>-<<<[-]>>->[>>>>>]+[-<<<<<+]->>>[-<<<[<<<<<]>>>>+[->>>>>+]-<<+>>>[>>>>>]+[-<<<<<+]->>>]>>-[<<<<<]>>>>+[->>>>>+]<<[...[-]>>->[>>>>>]+[-<<<<<+]->>>[-<<<[<<<<<]>>>>+[->>>>>+]-<<+>>>[>>>>>]+[-<<<<<+]->>>]>>-[<<<<<]>>>>+[->>>>>+]<<] 
Hãy thử trực tuyến!
Đầu ra cho trường hợp thử nghiệm 2: f ">,[>,]<[.<]",",[...,]"
>>>>>->>>>->--<<<>>>>>>>[+]-<<,[>>>>>>>[+]-<<,]<<<[+]-<<[>>->+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]--<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>+<<<<<[+]-<<]>>[>>>>>]<<<<<[+<<[-]<<<]>++[-->>>>>++]+[-<<<<<+]->>>>>-<<<[-]>>->[>>>>>]+[-<<<<<+]->>>[-<<<[<<<<<]>>>>+[->>>>>+]-<<+>>>[>>>>>]+[-<<<<<+]->>>]>>-[<<<<<]>>>>+[->>>>>+]<<[...[-]>>->[>>>>>]+[-<<<<<+]->>>[-<<<[<<<<<]>>>>+[->>>>>+]-<<+>>>[>>>>>]+[-<<<<<+]->>>]>>-[<<<<<]>>>>+[->>>>>+]<<]
Hãy thử trực tuyến!
Đầu ra cho trường hợp thử nghiệm 3: f ",.",",."
>>>>>->>>>->--<<<,>>->+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]--<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>+<<>>[>>>>>]<<<<<[+<<[-]<<<]>++[-->>>>>++]+[-<<<<<+]->>>>>-<<<[-]>>->[>>>>>]+[-<<<<<+]->>>[-<<<[<<<<<]>>>>+[->>>>>+]-<<+>>>[>>>>>]+[-<<<<<+]->>>]>>-[<<<<<]>>>>+[->>>>>+]<<.
Hãy thử trực tuyến!
Trường hợp thử nghiệm phức tạp: 
Đoạn A: Xây dựng tam giác bảng chữ cái >+++++[<+++++>-]<+[>>[>[.>]]>++++++++++.--[<++++++++>-]<[+.<]<-]>>,>[.>]++++++++++.[<[.<]>,>[.>]<] Hãy thử trực tuyến!
Đoạn B: Sắp xếp đầu vào theo thứ tự tăng dần >>,[>>,]<<[[-<+<]>[>[>>]<[.[-]<[[>>+<<-]<]>>]>]<<] Hãy thử trực tuyến!
Kết quả:
>>>>>->>>>->--<<<>>>>>>>[+]-<<+++++[<<<[+]-<<+++++>>>>>>>[+]-<<-]<<<[+]-<<+[>>>>>>>[+]-<<>>>>>>>[+]-<<[>>>>>>>[+]-<<[>>-<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>>+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]-<<>>>>>>>[+]-<<]]>>>>>>>[+]-<<++++++++++>>-<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>>+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]-<<--[<<<[+]-<<++++++++>>>>>>>[+]-<<-]<<<[+]-<<[+>>-<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>>+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]-<<<<<[+]-<<]<<<[+]-<<-]>>>>>>>[+]-<<>>>>>>>[+]-<<,>>>>>>>[+]-<<[>>-<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>>+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]-<<>>>>>>>[+]-<<]++++++++++>>-<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>>+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]-<<[<<<[+]-<<[>>-<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>>+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]-<<<<<[+]-<<]>>>>>>>[+]-<<,>>>>>>>[+]-<<[>>-<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>>+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]-<<>>>>>>>[+]-<<]<<<[+]-<<]>>[>>>>>]<<<<<[+<<[-]<<<]>++[-->>>>>++]+[-<<<<<+]->>>>>>>>>>>>[-]>>->[>>>>>]+[-<<<<<+]->>>[-<<<[<<<<<]>>>>+[->>>>>+]-<<+>>>[>>>>>]+[-<<<<<+]->>>]>>-[<<<<<]>>>>+[->>>>>+]<<[>>>>>>>>>>[-]>>->[>>>>>]+[-<<<<<+]->>>[-<<<[<<<<<]>>>>+[->>>>>+]-<<+>>>[>>>>>]+[-<<<<<+]->>>]>>-[<<<<<]>>>>+[->>>>>+]<<]<<<<<<<<<<[[-<<<<<+<<<<<]>>>>>[>>>>>[>>>>>>>>>>]<<<<<[.[-]<<<<<[[>>>>>>>>>>+<<<<<<<<<<-]<<<<<]>>>>>>>>>>]>>>>>]<<<<<<<<<<]
Hãy thử trực tuyến!
               
              
Shortest code in each language winvàShortest Brainfuck solution will be acceptedlà hai tiêu chí chiến thắng khác nhau.