Implementing multiple mappers on a cart would be a difficult (or perhaps impossible) task. Only one mapper was used at a time on a cart as there was no need to be so insane. It is expected/recommended that the bitpack for an OHB specifies which expansion chip to use for an NSF+ compo, unless there is some other strong theme to keep the entries coherent.
Accepted file format
Playback (for voting)
Apart from the tools you can use for creation, also:
- NSFplay and VirtuaNSF (version 188.8.131.52 is recommended)
Render to MP3
Since NSFplay and VirtuaNSF are both capable of emulating expansion chips, rendering an NSF file with expansion chips is the same process as rendering one without. See here for details.