The RTCM has a single connections for a receiver and transmitter, however they cannot work independently.
[....] if you would like to have the 2m and 6m repeater separate from each other you will need 4 RTCMs. 

If you configure the two AllStar nodes as duplex without local repeat, I believe you can tie the nodes together where one feeds the other and vice versa - a pair of split site repeaters using just 2 nodes and 2 RTCMs.