How to achieve concurrency in spidermonkey (handle multi instances | global object | scopeChain)

Hello Everyone,
   my application handle multi calls in the same thread, each call uses spidermonkey to run javascript code and as a container for variables to set and get values from it, the problem is how to separate between these spidermonkey engines. The calls also defined in the heap as we don't know the number of calls.

any advice will be most appreciated.