You can add users from multiple domains providing you have a trust between the domains. Ideally you'd have a two-way trust; it is possible to make this work with a one way trust (with the domain that the CRM server is in trusting the other domain), but this
prevents CRM from reading data from the other domain.
The are two alternatives, but neither are compelling. You could move all users into the CRM domain, or you could create two separate, independent CRM instances.
Domain trusts are normally straightforward to setup. You do need to have the domains and forests at a certain functional level (Windows 2003 may be sufficient, or Windows 2008 is fine)
Microsoft CRM MVP - http://mscrmuk.blogspot.com/ http://www.excitation.co.uk