I am not quite sure if it is possible. I have set different domains at the same G apps accaunt, and it is easy, just add another domain and authenticate it.
I'm pretty sure there is no easy way to do that from the end-user GUI. If you have paid plans you could try to contact Google support and ask if they can do it for you (but I'm not hopeful, given Google Support's reputation).
What you could do is to download all mails to your local PC using Outlook, and then upload them to the other account using Email Uploader. Note that Email Uploader is being retired, but the link to the new replacement, Migration for Microsoft Outlook, does not seem to work right now.
I uploaded mails to Google Apps once, and it worked well enough, but it was a bit tricky to set up as I remember -- and it took a long time, 24 hours plus.
If Google Support can't help you, then the above trick is a bit difficult -- can't you just keep both accounts operative, and forward emails from one to the other from now on?