The CAB2 will run anything that runs with the old CAB1 with a single exception, that being the old TMCC PowerMaster. If you simply desire an extra remote, consider the CAB1L, it's the same form as the CAB1 with updated electronics that will talk to the Legacy command base.
If you don't now own the old TMCC BASE1/CAB1, there's no reason to try to combine it with Legacy, time to move on.