Heard you guys were thinking of trying this; we have been doing this in the Northside Chain gang for a couple of years now and it works really well. The one bunch tends to be a core of 6-8 guys working and alot of guys sitting on which does make it more dangerous. The split groups encourage everyone to either want to make the catch or stay away from the faster group so folk tend to work harder as they feel more part of a TTT than a race bunch. Rolling through with folk with similar ability also makes it both harder, safer and smoother rather than sprinting or unable to close gaps.
It is also good to mix up now and again and set scratch off first, this encourages group 2 guys who think they could step up to try the scratch and if they get dropped they can join back in with second group- and similar for group 1 and 2.