Miles and cash for different legs
 

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.

Wirelessly posted (beckoa's BB: Mozilla/5.0 (BlackBerry; U; BlackBerry 9810; en-US) AppleWebKit/534.11+ (KHTML, like Gecko) Version/7.1.0.694 Mobile Safari/534.11+)

Many carriers offer OW awards, so this strategy may work. But be aware, especially internationally OW paid tickets tend to cost much more, negating the savings miles may yield.

I think you'll find that one-way international tickets are prohibitively expensive, so that strategy may not work. You'll generally get a better price buying a round trip. You also have to consider that award tickets are capacity controlled and may not be available on the days you want to travel.

If I redeemed them for a RT fare, would I still buy the tickets for both people at the same time, or would it be two separate transactions (one for the miles redemption, and one paid with cash)?

If you book them on line yourself, the airlines I fly will make you book them separately. You might be able to put one on hold while working on the other--depends on the carrier you are booking with. AA, for instance, will allow a 24 hour hold on paid tickets and even longer on award tickets. It's going to vary some between carriers.