Hi JMoriner,
I rented a single disc grinder from Sunbelt a couple of weekends ago. Great bunch of people to work with! See this thread for pictures of the grinder:
http://www.garagejournal.com/forum/showthread.php?t=125318
I had to also rent the 'Dyma-Serts' - you have to have three for the single disc and 6 for the double. You can see them installed on the machine held in with wooden chocks (that come with the rental).
I found the single disc pretty easy to work with. It took me about 4-5 hours to do a single pass over ~1000sqft. The following day I brushed the whole floor and concentrated on the bits I had missed, the edges and low spots (another 3-4 hours). I rented one of their shop vacs as well, which had some effect but not much - most of the dust seems to be thrown out radially from the head rather than being sucked up by the vac. I would recommend it though - I had more than a few inches of dust in the vac by the end, but about a trash bag full of dust that was swept up by hand!
I'm not sure about the double disc, but I used a 'side to side' motion with the single disc which seemed to work well. I also had the limitation of being able to lift and transport the thing in the back of a station wagon.
I managed to get pretty close to the edges (within 1-2"?) with the single disc. My floor had a build up of concrete at the edges which meant the head would get caught so I had to be a bit careful. I've been finishing the edges by hand this week (see above thread).
It goes without saying but I made sure I wore a good dust mask, and ear defenders (with ipod, it's pretty monotonous work!) I also wore a hat the second day - that dust gets *everywhere*!!
Good luck - post some pictures when you get around to it.
Cosmic