Seconding rewarding for stepping down. And sometimes randomly rewarding for staying put.
I have multiple birds and train them simultanously, going from one to the next to the next. Patience training is sort of baked into this approach, as they pretty quickly learn to wait their turn, and that out-of-turn behavior usually won't bring treats.
I say "usually", because rewards or lack thereof depend on the interference factor.
Sometimes one or the other will jump the gun. My general rule in that case is:
1) If the eager beaver interfered with the other to where they couldn't do the trick (e.g. calling one, but another comes, therefore blocking the called one from landing in the desired spot), the line jumper gets told to wait their turn and put back with no treat. The one who got messed up gets another chance to try.
2) If it doesn't interfere with the other's training (e.g. I ask one to wave and they both do), then they both get a treat.