What I really want is a JavaScript API--doesn't need to be a "VM": just a wrapper for an existing one--that makes it trivial to manipulate JavaScript engine spaces but where instead of them merely being separate memory allocators (as would be the case if you allocated two JavaScriptCore runtimes or engines or whatever they were called) the code is run in a separate process that doesn't contain any others memory or information and all communication with it is done via some kind of IPC (which you would then minimize using).