The goal is to have them a bit more than 100m apart. During observations they aim to maintain millimeter precision.

I'm pretty sure the risk of collision has been analysed to death. I would expect they've analysed what would happen if one or both devices suddenly stop listening to commands, and that even then there's essentially no risk.

The article says they're going to be 144 meters apart. A collision of things that are in basically the exact same orbit is also not going to be much of a risk, as the collision would be at a low relative velocity. Collisions between satellites are a kessler syndrome risk because due to differing orbits, the collision velocities can be in km/s, which can spread debris into a wide range of orbits, thus causing the heightened risk of cascading failures.

It still doesn't matter according to physics. There's a reason why two NASCAR cars can be going 150+ miles an hour, and yet "rubbin's racin'," but a head-on collision at 60 miles an hour will kill you. The two race cars going 151 and 149 mph rub at ~2mph relative velocity, while the head-on collision with the drunk occurs at 120mph relative velocity.

And flying formation, be it in space or in the air, by definition involves getting as close to zero relative velocity as possible . . . or else you aren't in formation.