Q: Apple ID

So I get this straight, I will recap:

1. Apple ID #1 = well used Apple ID for everything - ASC, Stores, iCloud, etc.
   You want this Apple ID to have a preferred email address used in #2
   It now uses some other email address (email #1)
2. Apple ID #2 = an UNused for anything
   It utilizes the preferred email address(email #2) which you wish to recover use of for #1

Right?

If right, then:

1. Login to Apple ID #2 at https://appleid.apple.com/ with its current credentials = email #2
2. Change that email address to another email address = email #3
3. Save changes = this frees email #2 for use
4. Log Out of Apple ID #2
5. Login to Apple ID #1 at https://appleid.apple.com/ with its current credentials = email #1
6. Change that email address to email #2
7. Save Changes

That should do it.

Change your Apple ID - Apple Support

http://www.apple.com/support/appleid/manage-account/