Yes, you can install CUCM on two servers assuming you have the SW and your servers are compatible.
The network diagram for a lab is pretty much what you want to build/test.
In mine I just have the servers and phones plugged into a 3560 and a 3524 which has a trunk between them.
Router on stick as I have UC apps on 10.x.x.x network and ip phones on 192.168.x.x
GWs also on 10 network and a few PRIs between them.
HTH
java
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www.cisco.com/go/pdihelpdesk
HTH
java
if this helps, please rate