You cant combine two AdobeIds that you used for ebboks once you've created them.
For other - the easier thing to do is to change the email address on your AdobeID, istead of creating a new AdobeID.
This subject comes up often, and if you do some searching on the Forum,
you'll see a lot of different comments. The bottom line, though, is that
the ebooks are not transferrable from your old email address to the new
one. If the ebooks were purchased, there's a chance that you can go back
to the seller and discuss the situation with them, and they may give you
another download to your new email address. If the ebooks are in the
public domain, you can go back and download them again to your new email