It is possible to have more than one SICStus run-time in a single process. These are completely independent (except that they dynamically load the same foreign resources; see Foreign Resources and Multiple SICStus Run-Times).
Even though the SICStus run-time can only be run in a single thread, it is now possible to start several SICStus run-times, optionally each in its own thread.
SICStus run-times are rather heavy weight and you should not expect to be able to run more than a handful.