I was speaking of adding server-client type facilities that specifically address multiple games running in one instance. I'm just going to leave it be, one game per engine instance, for the foreseeable future.
It's true that it does happen (I do the same thing, testing server-client functions without worrying about the network, using two machines, etc.). But having two games share the same backend (more like how a web browser works) would be tremendously complicated. Simple in theory--but it would be a LOT of work to implement, and would cause at least some overhead when only one game is running.
Running two instances of the engine at once is still quite possible, though. That should always be possible.
Considering a debugger, it would be fairly simple to make a rudimentary piping plugin for TurboSphere that would let a second instance examine the JS stack on another instance.But V8 already has GDB debugging integration. Or so they say--TurboSphere is always distributed with a V8 library that has that enabled. But there is, as with almost everything Google does, no documentation on how it works or how to use it.