I ride an 800 RMK Pro for the the same purpose. Your 600 will do great. For double-up riding, you will want to ride the sled 'rodeo' or at least that's what we call it.
Not my picture, but you get the idea. One person on the left side has brake control and the person on the right has throttle control and both hold on to the mountain hoop. Lots of leaning and teamwork required, but is by far the easiest and fastest way to the top of the hill. We rarely skin anything in our group. You shouldnt ride 2-up on a pro because of the lightweight tunnel...you run the risk of bending it.
If there are only two people riding we always use two sleds. One sled gets staged at the bottom and we ride together to the top. Ski down together, never by yourself. (remember, always have a buddy when you ride for safety) Then we reset the sleds and so on. Four people can do two sleds with two always driving and the other two skiing. Odd numbers get complicated but are doable.
Take your time putting a 'road' in for going up and down the hill with just one person on the sled. The smoother and straighter the better. It will firm up and make doing laps a ton easier and faster... riding two on a mountain sled is hard on them.
If I were only going to sled ski and not do any boondocking, I would certainly get a 2009ish M8 or a Rev Summit. Couple buddies have them and the are just strong sleds that work well for this purpose. Riding the sled is just as much fun though, so that's why I went with the pro.
