MP is linked to ticket numbers. Other than at the gate, they don't check photos, names, etc. So for instance on our last trip when I took an afternoon break, DD18 was able to use my ticket (via the app) to get an additional FP, either for the same ride twice, or for two different rides (one on her ticket, one on mine). But either way to answer your question, dad could MP one ride and you and kiddo could MP another ride. Or if you don't need your FPs for a time, someone else in your party could use them.
I know that technically letting someone else use your FP is not the way it's supposed to work. But I don't see it as any different than what used to be done in the past ... giving away FPs to total strangers if we weren't going to use them, or me getting a paper FP for DD to use before I left the park for the afternoon.
As others have said, though, if those two rides are the only thing you're getting it for, that would be a waste of $$.