There is a way to do this. When you configure Home Sharing in iTunes, you don't have to use the same Apple ID that is used to log in to the iTunes Store. What I did was the following:
1. Configured Home Sharing on Apple TV with my Apple ID.
2. Set up Home Sharing on my Mac with my Apple ID.
3. On my wife's Mac, authorized my account in the iTunes Store. (Not sure if this is necessary, but I did it.)
4. On my wife's Mac, enabled Home Sharing but with my Apple ID.
So, iTunes on my wife's Mac is logged into the iTunes store with her Apple ID but is connected to Home Sharing using my Apple ID. With this setup, Apple TV is able to see both libraries, and we can play content with either of them without reconfiguring Home Sharing.
Note that we are running Apple TV G3 with latest software (as of 2/16/13) and iTunes 10.
Hope this helps!