Users from trusted domain cannot use Direct Access

Hi,

I'm trying to get the following to work:

In Domain A we have a Server 2012 up and running with Direct Access, single NIC behind an edge. Users from Domain A are able to connect to Direct Access and its working as intended.

Now I want to be able to allow users from Domain B (there is a fulltrust between A & B) to access the Direct Access in Domain A. 

How can i set this up? I've tried to get it working but as of yet no success. In the logs i can see that the authentication of the Client computer is working as that is in Domain A but the user authentication fails, this is in Domain B.