I had the same problem as you with two of my sound mods : I was unable to use them together, the same sound was played for the two trucks.
I don't know if you make your sound mods like me but here is where my problem came from : I used the same project for the two mods (I just copy one project to have the second). Even if I created a new bank, I used the same events in FMOD studio. And since it was somehow the same events for two projects, there may be some kind of conflict between them in game thats why there was only one sound played. To fix it, I just duplicated the events of one project in order to generate new events with the same sheets, sounds, setting, ... and the I deleted the old events. Now I can load my two mods together in game without any issues.
I hope it can help you because your mods are very nice but it's a bit annoying not to be able to use them all at once.
