[isalist] Looking for pitfalls

I would like to indulge the minds of ISA List on the pitfalls of having two separate networks/domains behind one ISA 2006 firewall.
The main question: How does authentication in ISA 2006 work with two domains?
Any thoughts would be greatly appreciated - I should probably rephrase this ;-) 

Scenario:
Both domains are Windows 2003.
Both domains have Exchange servers publishing OWA etc...
Both domains have users requiring RDP and VPN access
All users except admins are not allowed into opposing network

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