Hi!
How would you synch? A semaphore, or a built-in like I believe there's in Java? In my programs I put this way, the threads per se don't know about each other, but share variables likes mutexes. And I'd create one diagram per thread. They'd use the same mutexes in the control flow... Would you like to have all in a single diagram?
I haven't used timing diagrams, but I think it wouldn't be that useful for multi-thread, because you can't predict when threads will colaborate. Not sure but I think timing is more to real time systems. But I'd like to listen if otherwise...