It is industry standard for coaches who are working the same sessions of a meet to carpool. But it should not be expected for, say, a coach that is working an 8am to 12pm session and a coach that is working a 1pm to 5pm session to carpool and sit around while the other coach works.(my understanding of what op describes is happening) As coachp and others said, either both coaches should be paid for both sessions, or both should get mileage for driving separately.
I guess I'm confused because normally this isn't an issue for local meets since the carpooling doesn't matter so much. When you're traveling somewhere requiring a hotel stay, I guess maybe this is an issue if your gym books hotels really far from the meet venue, but the way we do it usually no one has a car, so we stay close to the venue. If the set up does require you to be at the meet venue the entire day, I do think that's different and just sounds like poor planning to me (not that the coaches can do anything about it if they don't plan the travel). We do drive and stay overnight sometimes within say four hours but we've never really had an issue with finding a way to make it so people with sessions near each other can get back if it's a "driving" venue. Someone from another gym would also offer to give us a ride or we'll use the hotel shuttle, etc.