Like has been said.. Communicate. A person needs to know what parts they are playing so they are not standing all over each other. As to tempo, that's on the guitarist IMO.. If there are no unexpected curves to a cover - how hard is it to pick-up or slow-down to a tempo, or catch up to a missed beat etc.
Made me laugh, somebody mentioned volume.. ...I turned up to a practice one time with my usual, a Hughes and Kettner 100w combo, and the new guy we were auditioning shows up with a Marshall Half-stack. I stood not one chance!!
...Much like your good self, I just stepped back and let him carry for that first practice, then we talked about who was doing what and got our levels taken care of. It was kinda cool actually - he could play all the technical stuff much better then I ever could, but really stumbled on the slow stuff, so we kinda complimented each other. We ended up being a pretty-good team. I played my solos in my songs, his in his, he played the more technical solos in covers, me the slower stuff, that kinda thing. Really worked out. Broadened our ability to perform a wider variety of sets. That guy taught me a bunch of stuff too.
It all comes down to communication though. Except the tempo thing.. Not being able to adjust to tempo? That might worry me a little ...But, yes.. occasionally, doing what you did. taking a step back and playing a supporting role and stepping in when necessary.. is just doing what you gottsa do.