Here are a few rules to go by when on this subject:
1. Custom downloadable maps will have to be installed on both computers. If only u have it, only u see it.
2. Objects spawned by trainers: You can spawn 500 until the game crashes, only 70ish will be seen on MP. This works both ways, if your friend spawns 500, you'll only see 70ish.
3. If you both want to see a 350 object mega ramp that you just built, you'll have to save all your spawns to an ini, send it to your friend, have them drop it into their trainer directory and have them spawn the same thing.
General rule of thumb, if your in a MP game dont spawn too much intrusive stuff or you'll probably get kicked. If you have a couple ramps that lead 100 feet in the air to the megaramp, thats not a problem. However, if your mega ramp covers the entire ground of the airport, people are gonna be pissed. lol.
PS before you decide to leave a game, delete all your objects. They have been known to stick.