I'm new to using miles/points. I'm wondering if it's possible to purchase some legs of an itinerary with miles and other legs with cash. I understand that for a given leg, if I need 30k and I only have 25k then I need to earn/buy the other 5k miles first—my question is different than that, I think.

For example, suppose I want to fly RT from NYC > FRA for 2 people. That's essentially four tickets: NYC > FRA for person 1, NYC > FRA for person 2, plus the return tickets for each person.

1. Can I use miles to pay for one of those tickets and pay cash for the other three?
   
2. If I have enough miles for two tickets, should I book one person with miles and the other person with cash, or purchase the outbound flight with miles and inbound with cash, or does it not matter?

This could be on UA, LH, SQ, BA—I'm still working out where my miles would be best spent.