Sailing the M18 with double trapeze is possible and the platform can handle it. I've sailed my A-class with a combined weight of 125 kg and single trapeze and noticed that the mast top flexed too much and you lose a lot of power from the sail. Mitch Booth made a F18ht out of a M18 ... The platform was ok but the boat seemed to be a bit low in the water. If you keep the crew weight below 140 kg you will probably get it high enough in the water but you need a stiffer mast! Try a F18Ht mast and sail maybe?