Node.js only has a single thread for the event loop. (where the user's javascript is run) Internally it uses a thread pool.
You could certainly do what you describe and it would be very similar to node's Cluster module, which uses multiple processes. In fact, I believe in the next release they're giving you the option to replace processes with threads under the hood.
That actually isn't the case anymore. It has been removed for various reasons, a major one is stability. While the promise of isolates was nice, I think the choice to remove them was a good one considering the tradeoffs.
You could certainly do what you describe and it would be very similar to node's Cluster module, which uses multiple processes. In fact, I believe in the next release they're giving you the option to replace processes with threads under the hood.