I think I have two Apple ids on different products. Can I merge them?

I purchased many songs on my Mac Air. Then I went to my Mac Mini to download the songs to my ipod. The songs were not on the Mac Mini, but I was charged for them. I noticed I have another apple Id on the Mac Mini, but do not know how to merge the two computers. The lap top is new to me and very frustrating. Both have Lion if that helps solve my puzzle.