One way to do this would be with CNODES.

Assume at a particular node (say 500) this is where you will break the model into two parts. Instead of using node 500, use node 499 on the left-hand part and node 501 on the right-hand part. For the "entire model", define a restraint at node 499 with a CNODE at 501 and set the restraint type to "anchor". When you want to analyze the parts separately, just delete that restraint.