Ah I know what you mean. That was my fear when I brought my two boys to the vet because they rumble a lot, but they got along completely fine going to and from the vet and there were no problems. If they just rumble at each other and don't fight while you're cage cleaning, they shouldn't fight on the way to the vet either
Especially if you put some hay in the carrier, that should distract them and keep them happy during the car ride (although mine were very quiet and unsure on the way there and just huddled up and didn't eat any hay for a while until they had calmed down)