Seems to have been a while since we had a black hole thread 
As far as I know, the current theory is that if two black holes collide, you get a ton of gravitational waves, and a single hole with an event horizon of size at least equal to the sum of the two constituent holes’ horizons.
That’s fine, but I always thought that from an external observer’s point of view, time slows down asymptotically for objects falling into the hole. If I watch you fall into the hole, I actually see you fall slower and slower until you hit the even horizon, at which point I see time “stop” for you (even though in your frame of reference you experience falling “into” the hole in a small finite time.)
I’ve even heard it said that were you to visit a black hole, you’d see a ring of debris and trapped light around the outside.
How does this play out with colliding holes? What would I see, assuming I’m at a safe distance or have some magical force field? Would I just see the holes asymptotically approach each other, getting slower and slower? If that is the case, how can we hope to receive gravitional waves from the event in our frame of reference, when subjectively, for us, the collision even never actually happens? 
As an aside, what’s the current thinking on what happens inside the event horizon? I know the space lines become timelike or something, and “you can’t avoid the singularity any more than you can avoid next Tuesday because it lies in your future”, but that doesn’t really explain what happens. Any reasonable theories about this, short of quantum gravity (which AFAIK isn’t really a full-blown theory…yet)