connect two different email addresses to ADE

No, I don't think they can merge the accounts, especially if you can't even access the old account.

You need to set yourself up just to use the old account.

Sorry this is all a bit confusing below; it is confusing and I don't know how to explain it any clearer.

If you still remember the password for the old account you can still log in at the Adobe website using the old email;

even though it may not still be a valid email it is still a valid userid to access your Adobe account.

If you don't have access to your old email and don't remember the password, try Adobe Live Chat (~~~1 below)  to see if they can recover your access to the old account for you.

Having accessed the account you can change the email associated with the account to your new email, (but see ~~~2 below)

and then deregister ADE from the new account (ctrl-shift-D, cmd-shift-D on Mac), and register ADE with the old account (using its new name).

You will then be able to read your old books;  but not any books you have on the new account, I hope you don't have any yet.

If you do have bought books you want to keep on the new account already, see ~~~3 below.

~~~ 1

Adobe Live Chat: http://www.adobe.com/support/chat/ivrchat.html,

or as a slight short cut try http://helpx.adobe.com/contact.html?product=digital-editions&topic=using-my-product-or-service

Choose topic ‘'Signing into my Account' , and then click on 'I still need help';

then you should see 'Chat with an Agent' at the bottom of the page.

Depending on screen resolution, you may need to scroll down a bit to see the Chat with an Agent' bit, just under 'Ask our Experts'.

'Ask our experts' will indeed just lead you back to this forum.

Sometimes you will get ‘Sorry! All agents are busy— please check back soon.’

Don’t refresh the page, just hang on and it should eventually go to ‘Chat Now, and agent is available’.

~~~2

You can change the email used to identify a particular Adobe account, as I said above.

However, you can't have two accounts identified by the same email (it wouldn't know which one you were trying to identify).

If you have already created a new account using your new email, you can't immediately associate the old account with that email.

Many email providers allow variant email addresses. 

For example if you are 'fred@gmail.com', then 'fred+adobe@gmail.com' will also arrive at your 'fred.gmail.com' account.

So then you can change the email address on you old Adobe account to 'fred+adobe@gmail.com'.

Google will treat the two email addresses the same, but Adobe will see them as different.

You will now have old (useful) Adobe account  'fred+adobe@gmail.com',

and new (useless) Adobe account 'fred@gmail.com' which you can forget about.

Slightly more complicated to start with, but may save confusion later,

log in to your new Adobe account (using 'fred@gmail.com') and change it to use the email  'fred+deadnow@gmail.com'.

log in to you old Adobe account (using 'old@email.com') and change it to use the email 'fred@gmail.com'.

~~~3

The only way I know round this is to use a DRM stripper such as epubee BEFORE you deauthorize the device.

I do NOT advocate using such code to get around the valid limitations placed on a DRM book.

However, where the problem arises only because of the ineptness of Adobe's ADEPT DRM infrastructure and its implementation,

such a measure is totally justifiable.