It would be nice If you could set up several people in one simulation, to be able to simulate the interactions between classes
We already support this. In terms of the GUI interface might mean a little mucking around but just listing multiple profiles and simulating them all (Be sure to disable optimal_raid) would what you want.
That's pretty much how the Raid_T13H.simc profiles work. We create a profile that lists all our various specs but for the purposes of the Sim they're just all players in the same raid group attacking the same target. We do by default keep on optimal_raid=1 which just ensures that all the raid buffs are covered even if the raid is missing that particular buff. Turning off the optimal_raid=1 would ensure each class cast its buffs etc. instead.