User Authentication Remote Active Directory

Version 11.6.1

Active Directory with remote role groups is the current method for administrative access to the appliances. That is working for one domain within the forest. Due to a merger, now there are administrators in another AD forest (not just another domain in the same forest) that need access to the appliances. I am pretty sure that the only real methods I have in this case are create users in our AD forest or change to a different authentication method entirely. However, I thought I would ask the big brains of DevCentral. Has anyone else tackled this kind of situation? If so, please share your strategies.