Suppose we had two particles with a high kinetic energy travelling through the universe towards one another. They are pretty far apart from each other so the collision occurs very far away into the future.

Initially they had enough kinetic energy that if they collided near that moment, they would have formed a black hole. However, since the expansion of the universe will reduce their momentum and make them approach the hubble speed, would they still have kinetic energy when they collide? Or would it be much weaker and not form a black hole in any way? (Of course ignoring other interactions that would make them lose energy like friction, gravitational interactions...)

What I'm having trouble with is that, on the one hand stress-energy is locally conserved but on the other hand expansion makes the objects lose kinetic energy relative to comoving objects and "forces" it to approach comoving motion. So at the end, I don't really know what would happen in the collision of such particles. Would it be weaker than if two particles collide in a short period of time (where expansion has not decreased their momentum yet)? Would it have the same strength?

Concerning this, I have been told that this assumes that the objects are test objects--meaning their own energy is negligible. But of course if that's the case they won't form black holes if they collide--because their own energy is negligible. Wouldn't it work for particles with non-negligible kinetic energy?

I have also been told that in this case, if the particles are colliding with each other, the relevant energy is the total energy in their center of mass frame. The energy from comoving objects is only relevant if the particles collide with them. But, as the parricles would be very far apart from each other, wouldn't they be comoving objects themselves?