Can 2 Middleware environments share the same Siebel UCM 8.1.1 environment using the customer MDM PIP?

It is not feasible to link up the same UCM env to 2 Middleware environments without issues.  The UCM outbound services can be configured to publish events to only 1 Middleware environment at any given time.  So the outbound flows can point to only one PIP installation. If you intends to use both installations together, then outbound events won't happen on both Middleware environments, only one. If you are going to use it in turns, then that might work. Data corruption can occur if the same entity (Org/Person) is pushed to both EBS envs (somehow!).

For the flows inbound to UCM, well, technically it can be done. But here again, if it's the same account/person getting synced to the second EBS env, then data corruption will occur on the UCM end (the entity's cross reference COMMON ID is stamped on every record in UCM - if there are 2 Middleware environments, then this get corrupted).