We did the Rhino boats through Royal Caribbean a few months ago and loved it. Haven't yet seen where you can book other than the cruiseline, but that could of changed. Hope you do it and enjoy it!
Nope, doesn't work that way unfortunately. Two people go on and pay for two, but its well worth it, a lot of rhino riding before you get to your destination and you can trade off driving. I thought for the money it was well worth it, a nice way to see parts of the island you wouldn't ordinarly be able to.