V8 doesn't support (and actually blocks) concurrent access to a given runtime, but the application can spin up multiple runtimes and access them concurrently. Windows script engines are similar but more limited in that each instance must always be accessed from the same thread that created it.
In any case, it isn't ClearScript's goal to provide application infrastructure beyond script engine access. When it comes to concurrency, ClearScript doesn't dictate any specific model. Instead, it aims to make it easy to expose whatever model suits the application (or is already in use by the application) to script code, as long as it complies with the script engine's requirements.