Actually, we have a CD of the duo doing the covers that people liked, and then we have a CD of originals that my guitarist wrote (a studio album).

We split the cover CD 50/50, but my guitarist paid me (and many other session musicians) for the work when the original music CD was made (I also mixed and mastered it), so the proceeds of that one are entirely his, and well deserved!

The main thing about including originals is, if you are trying to make CDs for profit, rather than just to get your original stuff out, only do it if they are at LEAST as popular with your audience than the covers. To maximize sales, you want to put the most popular material on the CD you can, and be honest with yourself... if you get few requests for your originals (even if they don't clear the floor!), you get better sales sticking to the popular covers.

You can always do an original music CD as a labor of love, but it is probably better to have a separate CD rather than just do one or two in with the covers.. (unless your audience demands it!).