Docsis 1.1 and 2.0 in the same star

Assuming you mean same network in terms of both cmts' on the same (Example) /24 network.

You cannot do this, as routing won't allow it. I mean, you COULD do this, but you will have IP conflicts.

You have the (example) 10.10.10.0/24 network, .0 is the network address, .255 is the broadcast address, .1 or .254 would be the gateway. (hosts = N-2, or 254 in the case of a /24)

Your routing to CMTS1 would be to 10.10.10.0, therefore CMTS2 cannot use that network. You would have to set up (example) 10.10.11.0/24.

Hope I am understanding your question correctly.