Music made for the 2A03-chip and one or more expansion chips, that were featured in the Nintendo Entertainment System and its game carts. allowed expansion chips: VRC6 VRC7 MMC5 FDS N163 S5B
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
.nsf
Playback (for voting)
Apart from the tools you can use for creation, also:
- NSFplay and VirtuaNSF (version 1.0.6.1 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.