For streaming from other devices it is only the Home Sharing account, that matters. I have a third appleID set up just for that on all our family devices.
The ID that aTV uses to sign in to iCloud and iTunes Store only affect the content visible in aTV user interface - My Music, Purchased Movies, iCloud Photo Library and the like. And these account credentials can all be entered simultaneously and become switchable. But the content belonging to more than one Apple ID can not be seen at the same time in UI, I'm afraid.