Same user, two accounts, one mailbox

Hello everybody,

I need help resolving a very specific matter.

I need to have a user, who is a domain admin, to have another account that is part of the domain users group, but both accounts need to share the same mailbox.

How does one do this in ADUC?

He needs to be setup this way to be able to use a Palm phone with Good Technology client software installed in it.

Thank you in advance for any replies.