SX7: From what I have read, the dual runners will help eliminate darting, but proper setup will also cure the problem (for alot less money). You could start with vmaxjohn's article on the tech page (http://www.totallyamaha.com/snowmobiles/aaTECH/darting.htm), and if that doesnt help, just do a search on darting here on the board.
I tried the 4" carbide doolly's and took them off after the first year. The reduced darting was excellent, but forget about aggresive cornering. Maybe the one's with more carbide would be better but I dought they would carve as good as the single cabides. I have 144 picks and I am going to try 10" carbide Shaper Bars this year. I hate the darting but I hate hogging the centre of the trail around corners more.
local yammi dealer said some of his customers just run em on one ski......imo....they should be made with the outside carbide a little higher....that would give yu bite in the corners....but then again they would wear uneven....im gonna try the one ski option on my new vector....ill let ya know how it works. i think they prob work better on a sled without picks??
all ive heard is that peple around here really like em