Can Apple Macs bind to two domains in two separate forests?
In OS X Mavericks

The same as binding the Mac to the Active Directory to the Domain for the first time - System Preferences > Users &amp; Groups > Login Options. Unlock the advanced options and click Edit.

Here you can click the plus and add the additional domains' details

Some points to consider:


The Mac will need a computer account in the additional domain
Users who login where a user with the same samAccountName will be prompted for Admin credentials in order to rebuild the Library. This only happens when a matching user has already logged in to the Mac
Users will be prompted for the 'matching' user's password as OS X seems to try and migrate the users 'keychain'
Consideration should be given as to the order with which domains should be checked for authentication requests


I've still yet to dig into policies and configuration across the domains and how they affect the Macs' being in both domains.... though I still haven't learnt how that works so it may be a non issue