interleave.bf

[>]>[>]< to the end of the second string
[[>>+<<-]<] shift it two spaces right
>>>[>]< to the end of it again
[ spreading loop
 [[>+<-]<] shift the whole string to the right
 >>[<+>-] pinch the first char out of it
 >[>]< to the end of the string
] spreading loop ends
<[<<]<< to the end of the first string
[ interleaving loop
 >->>> to the second string first char (and leave a 255 beacon)
 [[<<+>>-]>>] copy the whole second string closer
 +[-<+] to the beacon
 <[>+<-] copy the lastmost char of the first string to the right
 >[>] to the end of contiguous segment of interleaving
 >[[>+<-]>>] copy all the spreadout cells to the right
 <[<<]<[[>+<-]<] to the contiguous segment and copy it right to free space
 < to the next last first string character (if any)
] interleaving loop ends
->>> to the string again (and leave 255 beacon)
move it all back to the starting positions:
[[<<+>>-]>] first the contiguous part
>[[<<+>>-]>>] and then the spreadout part
+[-<+]> to the beacon (and we're done)