I was imagining them flying close-by, so any failure would have a higher chance of causing a cascade. I couldn’t find any info on how close they are, but I imagine it’s too far for this to be an issue.
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.
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.