NO It REALLY depends on the CM.
Some CM's will not allow three people to sit in one car on an attraction like HM (the nemo ride, PP, etc) (i.e. two adults and one small child), even if everyone is fairly small!
Then again, I don't see what the big deal is...
If you are an adult asked to ride in your own cart, you simply ride in the next cart. You'll only be about ten feet apart max, on ANY attraction for the total duration of the ride. The attraction is only a few minutes long, and you'll be able to see eachother again as soon as the ride is over.
If you have a child who is a little scared, then maybe it will pay to decide ahead of time which adult is the priority to ride with the child. you can also load the single riding adult between the other cars.
Otherwise, it's REALLY not worth any extra consideration. Really. I'm not trying to be snarky or unsympathetic, I just don't see that this would be an issue. On most attractions, you can SEE the cars around you for much of the ride, and most rides are under 5 minutes long.
UNLESS - the adult who has to ride alone is disabled. If that's the case, then Disney has ways of helping. Post some specifics, and we can better heklp you.