I don't really understand why you need 2 GPSs to be able to see stats and navigate. The Edge series machines have a number of pages (mostly customisable) which can be scrolled to give an astonishing amount of information, and navigation instructions will override any stats page when necessary. However, if you want 2, you want 2!
The Edge 810/1000 don't have waterproof USB connections, though I'm sure lots of people will rock up to say they've never had a problem. Both the 810 and 1000 connect to your phone via Bluetooth, so ride stats can be uploaded to Garmin Connect (and, via that, Strava, MapMyThingy, RWGPS, WHY) as and when you wish without the involvement of a computer. AFAIK, they can't connect to a USB drive for the purpose of downloading data. However, they function as USB mass storage devices when connected to a computer so the file system can be fully accessed if you really want to.