VBA, 512 489 479 bytes
Sub f(A,B):Debug.?">>>>>->>>>->--<<<"&Replace(Replace(Replace(A,"<","<<<[+]-<<"),">",">>>>>>>[+]-<<"),".",">>-<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>>+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]-<<")&">>[>>>>>]<<<<<[+<<[-]<<<]>++[-->>>>>++]+[-<<<<<+]->>"&Replace(Replace(Replace(B,"<","<<<<<"),">",">>>>>"),",","[-]>>->[>>>>>]+[-<<<<<+]->>>[-<<<[<<<<<]>>>>+[->>>>>+]-<<+>>>[>>>>>]+[-<<<<<+]->>>]>>-[<<<<<]>>>>+[->>>>>+]<<"):End Sub
Explicação
O código VBA altera o código do cérebro de uma maneira, para que a saída do trecho A seja armazenada em uma lista e a entrada do trecho B seja lida nessa lista.
Inicializa algumas variáveis
>>>>>->>>>->--<<<
Em seguida, ele lê trecho de A e substitui todos <
por <<<[+]-<<
, cada >
por >>>>>>>[+]-<<
e cada .
pela rotina de armazenamento
>>>>>>>[+]-<<"),".",">>->+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]--<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>+<<
depois disso, ele exclui a memória do trecho A e faz alterações na lista armazenada, para que possa ser lido como entrada para o trecho B:
>>[>>>>>]<<<<<[+<<[-]<<<]>++[-->>>>>++]+[-<<<<<+]->>>>>-<<<
O trecho B será lido, todos <
serão substituídos por <<<<<
, todos >
serão substituídos por >>>>>
e todos ,
serão substituídos pela rotina de leitura da lista:
[-]>>->[>>>>>]+[-<<<<<+]->>>[-<<<[<<<<<]>>>>+[->>>>>+]-<<+>>>[>>>>>]+[-<<<<<+]->>>]>>-[<<<<<]>>>>+[->>>>>+]<<
Código fonte do Brainfuck
Esta é a minha fonte para as partes do código do cérebro. Vou explicar isso em detalhes mais tarde.
[
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
Saída para o caso de teste 1: f ",[..,]",",[...,]"
>>>>>->>>>->--<<<,[>>->+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]--<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>+<<>>->+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]--<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>+<<,]>>[>>>>>]<<<<<[+<<[-]<<<]>++[-->>>>>++]+[-<<<<<+]->>>>>-<<<[-]>>->[>>>>>]+[-<<<<<+]->>>[-<<<[<<<<<]>>>>+[->>>>>+]-<<+>>>[>>>>>]+[-<<<<<+]->>>]>>-[<<<<<]>>>>+[->>>>>+]<<[...[-]>>->[>>>>>]+[-<<<<<+]->>>[-<<<[<<<<<]>>>>+[->>>>>+]-<<+>>>[>>>>>]+[-<<<<<+]->>>]>>-[<<<<<]>>>>+[->>>>>+]<<]
Experimente online!
Saída para o caso de teste 2: f ">,[>,]<[.<]",",[...,]"
>>>>>->>>>->--<<<>>>>>>>[+]-<<,[>>>>>>>[+]-<<,]<<<[+]-<<[>>->+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]--<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>+<<<<<[+]-<<]>>[>>>>>]<<<<<[+<<[-]<<<]>++[-->>>>>++]+[-<<<<<+]->>>>>-<<<[-]>>->[>>>>>]+[-<<<<<+]->>>[-<<<[<<<<<]>>>>+[->>>>>+]-<<+>>>[>>>>>]+[-<<<<<+]->>>]>>-[<<<<<]>>>>+[->>>>>+]<<[...[-]>>->[>>>>>]+[-<<<<<+]->>>[-<<<[<<<<<]>>>>+[->>>>>+]-<<+>>>[>>>>>]+[-<<<<<+]->>>]>>-[<<<<<]>>>>+[->>>>>+]<<]
Experimente online!
Saída para o caso de teste 3: f ",.",",."
>>>>>->>>>->--<<<,>>->+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]--<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>+<<>>[>>>>>]<<<<<[+<<[-]<<<]>++[-->>>>>++]+[-<<<<<+]->>>>>-<<<[-]>>->[>>>>>]+[-<<<<<+]->>>[-<<<[<<<<<]>>>>+[->>>>>+]-<<+>>>[>>>>>]+[-<<<<<+]->>>]>>-[<<<<<]>>>>+[->>>>>+]<<.
Experimente online!
Caso de teste complexo:
Trecho A: Construa um triângulo do alfabeto >+++++[<+++++>-]<+[>>[>[.>]]>++++++++++.--[<++++++++>-]<[+.<]<-]>>,>[.>]++++++++++.[<[.<]>,>[.>]<]
Experimente on-line!
Snippet B: classifique a entrada em ordem crescente >>,[>>,]<<[[-<+<]>[>[>>]<[.[-]<[[>>+<<-]<]>>]>]<<]
Experimente on-line!
Resultado:
>>>>>->>>>->--<<<>>>>>>>[+]-<<+++++[<<<[+]-<<+++++>>>>>>>[+]-<<-]<<<[+]-<<+[>>>>>>>[+]-<<>>>>>>>[+]-<<[>>>>>>>[+]-<<[>>-<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>>+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]-<<>>>>>>>[+]-<<]]>>>>>>>[+]-<<++++++++++>>-<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>>+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]-<<--[<<<[+]-<<++++++++>>>>>>>[+]-<<-]<<<[+]-<<[+>>-<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>>+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]-<<<<<[+]-<<]<<<[+]-<<-]>>>>>>>[+]-<<>>>>>>>[+]-<<,>>>>>>>[+]-<<[>>-<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>>+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]-<<>>>>>>>[+]-<<]++++++++++>>-<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>>+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]-<<[<<<[+]-<<[>>-<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>>+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]-<<<<<[+]-<<]>>>>>>>[+]-<<,>>>>>>>[+]-<<[>>-<<[-<+>>>>+[-<<<<<+]+[-->>>>>++]--<<+>>+[-<<<<<+]-<++[-->>>>>++]--<<]<[->+<]>>>>+[-<<<<<+]+[-->>>>>++]>>>>>-[-<<<<<+]-<++[-->>>>>++]-<<>>>>>>>[+]-<<]<<<[+]-<<]>>[>>>>>]<<<<<[+<<[-]<<<]>++[-->>>>>++]+[-<<<<<+]->>>>>>>>>>>>[-]>>->[>>>>>]+[-<<<<<+]->>>[-<<<[<<<<<]>>>>+[->>>>>+]-<<+>>>[>>>>>]+[-<<<<<+]->>>]>>-[<<<<<]>>>>+[->>>>>+]<<[>>>>>>>>>>[-]>>->[>>>>>]+[-<<<<<+]->>>[-<<<[<<<<<]>>>>+[->>>>>+]-<<+>>>[>>>>>]+[-<<<<<+]->>>]>>-[<<<<<]>>>>+[->>>>>+]<<]<<<<<<<<<<[[-<<<<<+<<<<<]>>>>>[>>>>>[>>>>>>>>>>]<<<<<[.[-]<<<<<[[>>>>>>>>>>+<<<<<<<<<<-]<<<<<]>>>>>>>>>>]>>>>>]<<<<<<<<<<]
Experimente online!
Shortest code in each language win
eShortest Brainfuck solution will be accepted
são dois critérios de vitória diferentes.