Hi,
For such issue, you may check the following Technet articles first. Hope it helps.
Design Active Directory for DirectAccess
http://technet.microsoft.com/en-us/library/ee382285(v=ws.10).aspx
Planning for Multi-site DirectAccess
http://technet.microsoft.com/en-us/library/ff625682(v=ws.10).aspx
In addition, in order to get best resource for this issue, I would redirect you to post this issue at UAG forum. You can get more effective suggestion by other experts familiar with this topic.
Forefront Edge Security – DirectAccess, UAG and IAG
http://social.technet.microsoft.com/Forums/en-US/forefrontedgeiag
Best Regards,
Aiden
Aiden Cao
TechNet Community Support
