Hi,
I'm afraid your assumption is correct - if you used different registrations (e-mail) for the Readers then eBooks bought on the other ID can't be read; you need a matching pair (so to speak) between the computer and the Reader. That said you should have been able to register both Readers with the same Adobe ID. I think Adobe allow something like 5 or 6 devices to share the same ID - my wife and I both have a Reader (same model), and we use a desktop and a laptop computer, and we've used the same ID on everything and all the eBooks we have are completely interchangeable.
The only thing I would say is that while it's possible to de-register and re-register your Reader (if that's what you wanted to do to read the older eBooks you have) any eBook you've bought will always be tied to whatever registration was used on the computer that was used to purchase it - so you would never be able to read both sets of books bought with different IDs simultaneously on the one device (hope that makes sense!)
If it's any consolation, there's a similar thing going on with Kindle - these are registered to your Amazon account, so if your daughter for example had her own Amazon account, then anything she bought wouldn't work on a Kindle you had registered with your Amazon account. I guess all companies/publishers are worried about copyright....:smileyplain:
Hope this helps.
