If two players try to do things simultaneously, how do we determine which one happens first?

If they were truly simultaneous and you can't agree on anything else, each player who wanted to do something should roll the die; high roll decides what order the cards happen. If that makes later cards moot, well, that's the way the game goes. (But: abilities that require discards don't happen until the discards hit the table. This is a good reason not to announce your plays until you are ready to make them!)