How do you sync your device to your on-line "library". And if you accidently have two "accounts" (don't ask), if you sync to one, does it erase,delete or remove items on the other account?
Turn on WiFi on the Nook and tap the sync button (double arrows in a circle). Your Nook can only be registered to one account at a time. If you deregister it and then re-register it to the other account, all the books from the first account will be removed from the Nook. Same thing is you re-register to the first account.
Your best option is to take the account with the least amount of books. Download Nook for PC and then download the books from that account and sideload them to the Nook.
Then only buy books on the other account.
If you don't know how to sideload the books via Nook for PC, follow the instructions in this post. The reason I recommend that you use Nook for PC to download the books is that you will get a somewhat recognizable filename, if you download from the website you get a file name with all numbers, plus it has an extra period in it and the Nook won't recognize it as a eBook. ex: 123445555.v2.epub.