Have you or someone you have known ever written a book & a screenplay together at or about the same time?

I have not known anyone personally to have done this, and doubt I ever will. That said, there is a first for everything. But I cannot figure out why you would want to do both at the same time. It is traditional to do the novel first if you intend to write both (not unheard of at all), but not at the same exact time. Reason being, a novel is very detailed, and a screenplay is very quick and concise. The benefit of completing the novel first would be that you now have the world's greatest platform to write a screenplay off of. You certainly would not include everything, but your characters would be well fleshed out and not flat, your story likely will be very clear, and you will understand what will work in a screenplay and what will not.