Nested in the UI is not possible I think?

Yes, but the next version will have nested projects, that's something 🙂

I mean that it is possible to start the subtask while the main task is still active.

You cannot call another Task.init while a main one is running.
But you can call Task.create and log into it, that said the autologging is not supported on the newly created Task.

Maybe the easiest solution is just to do the "sub-tasks" and close them. That means the main Task is not logged, but the others are ?