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