I'd guess that the main advantage of doing it as two parties would be to alleviate the need to have make separate trips into Scaleburn and Smallcleugh to rig or de-rig.
I've done the Scaleburn to Brownley connection twice now, once with a wetsuit once without. You'd be worrying without a wetsuit if things didn't go to plan but as long as you were wearing one the worse that'd really happen if you were waiting around or had to turn back is that you'd be fairly pi$$ed off.
Wholeheartedly agree re changing the route though, there's the route via J. Irwins as well.