It sounds as if you have done the right thing.
You can certainly have a single copy of ADE run with multiple eReaders,
as long as they are all three (computer plus two devices) registered with the same Adobe ID, and you only have one eReader device plugged in at a time.
(I'm not sure what happens with multiple devices plugged in at once).
There are bugs with ADE2.0 and some eReaders.
There are lots of bugs in ADE2.0. Try replacing ADE2.0 with the older but more reliable v1.7.2.
(You can have them both installed at once if you like.)
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.
http://helpx.adobe.com/digital-editions/kb/cant-install-digital-editio ns.html
The forum software is sometimes corrupting the link above. There shouldn't be a blank in 'editio ns.html'. The following redirects to the same page: http://tinyurl.com/diged172