I'm okay with giving a ride to one person and not the other
Well, OK. Why do you feel so strongly about this?
If I do both it will cost me more.
You can give a ride to who ever you want to. People should not expect you to drive them around without paying for it or offering gas money for your efforts.
I want to know who has more money.
Well, I guess you could ask. Like I said, it's up to you who you want to drive around.