From the Read Carefully dept.:
...A single SpiderMonkey runtime (that is, instance of JSRuntime) ??? and all the objects, strings and contexts associated with it ??? may only be accessed by a single thread at any given time. However, a SpiderMonkey embedding may create multiple runtimes in the same process (each of which may be accessed by a different thread).

That means it is up to the embedding to provide communication (if any) between the runtimes via JSNative or other SpiderMonkey hooks. One working example is the new implementation of web workers in Firefox which uses a runtime per worker. Niko Matsakis is experimenting with a different architecture in his new parallel JS project.