Sorry - you can't. You can keep the music from each iTunes account in the same library, but the purchased items will always be "owned" by the iTunes ID under which they are purchased.
My husband and I both purchase music and other stuff on his Mac in iTunes under our own iTunes IDs. Both IDs are authorized to the Mac, and everything is housed in one iTunes library. I can download his music, and he can download mine.
However, if, say, some of the music was removed from the library and we wanted to get it back in there, we would each individually have to go to our Purchased list to download the items that we purchased in order to get them all back into the library.
Hope that explanation helps!