If I book a flight (say, through Orbitz) on Aer Lingus that also has a codeshare with United, but I don't actually book the United codeshare flight number, can I still get credit for MileagePlus? Even though my ticket has only EI flight numbers on it?

Sorry if this is confusing.

And this is probably a really dumb follow-up. Again, if I book on say Orbitz, how do I choose which frequent flyer program the points go to? I'm also a member of the (albeit useless) Aer Lingus program. I want to be sure i get the MileagePlus credit and not Aer Lingus.

How do airlines stop it going to both?