>> so unless they collide they would just orbit each other.
That supposes blacks holes translate in space and would couple despite the momentum of one or both. Are the gravitational properties of two proximate black holes known or theorized?
The translation of interest would be translation relative to one another. As galaxies collide, it stands to reason that there are black holes moving relative to one another. The question is what happens if two (or more) come within "gravitational range" of each other, whatever that is.
As with any massive bodies, there is a possibility of a pass-by, slingshot or other effect. One thing known/theorized about black holes is that they have an event horizon. This is a location where the escape velocity (from the gravity of the black hole) is the speed of light. If something enters the event horizon, it's not coming out, no matter how much momentum it has.
The orbit mechanics are well known, but difficult to calculate in all but the simplest case. Newton, modified by Einstein. Black holes aren't special for orbital mechanics.