Allowing users on different domain to access mailbox

[Jambo_C]

I have two domains that are trusted. My Exchange 2010 server sits on my curriculum domain as most of my users are on this domain. I also have an admin domain. Each user has a mailbox and I have been able to get my admin users to access their own mailbox by converting the mailboxes to linked mailboxes and making the user on the admin domain the master account. However, I now need to set up some mailboxes that can be accessed by users on the admin domain going to "open" within Outlook from their own account. Exchange doesn't allow an account to be a master account on more than one mailbox so making it a linked mailbox is out of the question. I just need to add "full access permissions" to the account and be able to pick a user on the admin domain. Unfortunately this isn't possible. Does anyone know how I can achieve this? I hope that's clear and thanks in advance.