i make multichannel bcstms using this method, it's the only one i found and used so far but yeah here it is:
-a multichannel BRSTM converter (not mentioning the name because it contains a copyrighted nintendo program(?))
-a bcstm converter
you put the files on the wav folder using the following:
1.rename the first sound file as "1" (left)
2.rename the second sound file as "2" (right)
3.you rename the other files as "3", "4", "5", "6", "7" and "8" based on the channels the original bcstm have.
PS: every wav file needs to be in mono or else it won't work!
Now that you have all the 8 files, go to "multichannel BRSTM converter" and put the loop start and loop end and set the channels to 8 then press build batch file. When the batch file will be ready as the programs informs it go to the folder of that program, run the batch file (there should only be 1) and run it, after that it ended close it, go to output iirc and you will find your brstm, test it before converting.
Now that you got that multichannel BRSTM you are going to convert it using your preferred BRSTM to BCSTM converter and convert it.
There you go! you got your multichannel BCSTM! Have fun!
If there is an error in this tutorial let me know!