APM Auth to Multiple AD

Scenario is that we have different networks which use different Active Directory domains which are in separate forests with no trust between them.

I already have a Virtual Server on the APM platform which authenticates users to one of the Active Directories and based on group membership presents different user types with different WebTops containing different applications or Remote Desktops.

I now need to build another setup for the second network which authenticates to the second Active Directory and again based on group membership presents different user types different resources.

With Public IP addresses limited I would like to use a single Virtual Server/IP on the APM platform.

Ideally I'd like to configure the APM rules along the lines of "authenticate to AD1 if that fails attempt authentication to AD2"

Any thoughts/ideas?