Are there any predictions for what would happen if two black holes happend to cross paths? Would they just merge into one big black hole, or would something extremely strange happen? Or do we just not have enough data to know?
They merge into a larger black hole.
More specifically, the total area of event horizon for the two holes remains contant. So two smaller black holes merge into a bigger one that is less dense than the smaller ones were.
That’s as far as an outside observer could tell. What happens in their interiors is anyone’s guess since General Relativity can only give us infinities for what happens to the mass that falls in. I can’t even speculate on what an observer who has fallen past the event horizoni might witness on their way down.
Also the collision of two black holes would generate significant gravity waves.
I can: absolutely nothing. The tides would have ripped him to atoms before crossing the event horizon. 
"Event Horizoni, the pasta of cosmologists!"™
Pish tosh. I could cross the event horizon of a 10[sup]6[/sup] solar-mass black hole without breaking a sweat. It’s those small ones you have to worry about, since the tidal forces at the horizon are inversely proportional to the square of the black hole’s mass.
Correction: The total area of event horizon remains at least constant. In general, it will increase, and it will never decrease. This is closely related to entropy: In fact, it can be said that the area of a black hole’s event horizon is its entropy.
This is very correct. Black hole mergers are one of the major sources we’re looking for (currently and in the near future) using gravitational wave detectors. In fact, nearby black hole mergers are the only thing we really expect to be able to see using the currently-operating detectors (the fact that we haven’t yet seen any is therefore an indication that there aren’t many mergers near us). Because of this, there’s a great deal of work being done in the field to model the exact dynamics of black hole mergers. We can model their approach very well up to the point when they’re separated by a few times their radius, or so, and from the point where the horizons make contact, we can also model very well the “ring-down” where the combined horizons wobble down into shape. Unfortunately, the part in between is very difficult to model computationally using current methods, and we probably won’t know exactly how that part progresses until we actually see it happening many times.
Lumpy, I suspect that an infalling observer past the horizon would be completely oblivious to the merger, since he would reach the singularity (and therefore surely perish, regardless of the mass of the hole) before the effects of the merger could reach him. Both radial infall and merger occur on timescales comparable to the size of the black hole.
slight hijack, but what would happen if these black holes ran into each other:
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edge on but spinning at opposite directions. (I think that’s right. was told that the stuff falling in does so in a similar way as in a drain. Disregard if wrong)
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Can they do so end to end? I mean, do they have a ‘front’ and a ‘back’?