You would point the SG to a WA that would point the relative domain. If that is what you mean then that would get 1 Secure Gateway to multiple untrusted domains with broker in each domain...
I seem to recall you can specify multiple brokers/webaccess servers in SG gui but how would it work to direct to specific domains.  If you use the AppPortal client I guess it is configured to go to the correct broker..If you go to WebAccess then you may have to subdivide the url and have DNS point to each WebAccess server correctly.  My lab is not in a place where I can set up this scenerio...anyone seen this before?
1 Secure Gateway to multiple untrusted domains with broker in each domain