Different situation, but let me relay my experience.
Borrowed an electric golf cart which has a front bench seat and a rear facing raised bench seat. It's wheels were also "raised." Took it deer hunting with son and his family.
Bottom line, fully loaded it was way tippy, with wheels leaving the ground under conditions I did not consider "dangerous." We started out on a ride around with son and myself in front seats On the raised rear seat was his wife (slim), a petite female adult, and 6 yr old son. Half way thru the outing, I changed places with the petite female thus adding about 100 lbs to the rear and reducing the front load by about 100 lbs.
Son drove no more than 30 yards and noticed front wheels bouncing off ground...we carefully repositioned ourselves to the original seating arrangements and carefully went on our way.
Point is, I'd be worried about changing the balance of the vehicle and making it unstable if you were to put two 250 lb men in the raised rear seat and then drive over inclined terrain. Might not be a problem, but golf cart batteries are toward rear of vehicle, so is the engine of the RTV 900. Thus, a heavy load in the high rear seat combined with a steep incline, at some point, is unstable.
Just a thought.