You could probably approximate combo points, fairly reliably.

Either go when they will have at least four, but usually five combo points.
Or go with a finishing move when they all have five combo points, but some might have had five for a little while.
Not sure which would be the better DPS.

If you're using IS Boxer or something equivalent.
You could use video feeds, to see the current combo points for each rogue.
A clickbar could manage finishing moves for each rogue.
Although I'd probably lean more towards a more automatic timing with less management; even if some DPS was lost, it would likely be preferable compared to the amount of concentration/micromanagement required.