union of 2 PAs of RTC
One answer
If by "union", you mean move all of the work items from the 2 PAs into a 3rd PA on the same server, yes, you can do that, as long as the work item types in the 3rd PA are compatible with those in the 2 PAs. If you mean more than that (e.g., plans, builds, timelines, team areas, process), the answer would be "not without a lot of scripting".