Perhaps the primary reason to use more than one SICStus run-time in a process is to have each run-time running in a separate thread. To this end, a few mutual exclusion primitives are available. See Operating System Services, for details on mutual exclusion locks.
Please note: the SICStus run-time is not thread safe in general. See Calling Prolog Asynchronously, for ways to safely interact with a running SICStus from arbitrary threads.