In tug-of-war in official songs, the playable part flips back and forth between the two players, similar to face-off mode in Guitar Hero. If you play tug-of-war in a custom song, iirc both players just play the entire song start to finish.
How do official songs author the flip-flopping? I'm assuming it's just another marker, like in Guitar Hero, but I wouldn't have any idea what note they're on.
Here's an example of tug-of-war if you don't know what I'm talking about: