It's not too complicated. The sent sim is like an NPC or other townsperson. Yes the game does default to basegame or another as seen by the other player. The most complicated part is getting the timing right. Once you have sent out the simport request via the message area in play screen (upper right corner in game). Then the simport request shows up on everyone who friended you. There is a time period to accept the simport. Not sure if both player have to be logged on at the same time. It's a shorter window frame (not sure the exact frame but less than 24 hrs).
Anyhoo, I kinda like this feature, but as of late not as many peeps reliably online but if you see me, I have places if you need help. I'd like to get those sets too. I'm really getting into the different showtimes careers of late.