Sorry about the newbie question, but I can' find much with a search!

Q: How easy is it (generally) to combine miles from different airlines that are partners? The one thread I found about this makes me awfully pessimistic!!!

Specifically I want to take 11,000 Alaska Miles, and add 9,000 NW miles to get a trip on Alaska.

Also, I want to combing Midwest Express and Virgin.

I had 'assumed' it was easy to do. I hope I'm not screwed! Ugh...

Milo