I have slid them off a double trailer several times, by myself, and its quite easy. First, place the cart at the back of the trailer, and make sure that the carpeted 2x4's on the trailer and on the cart, butt up against each other. If they don't, shim up that end of the cart with some lumber, or if the cart is too high, lower the front of the trailer until they are at least touching. Now with the cart up against the back of the trailer, hook a cable come-a-long to the far end of the cart, down low where you have enough room to work the handle on the winch. I use a 12" piece of 1/4" chain wrapped around the axel and the center vertical support, where it won't slide up or down, or, side to side, and connect the ends of the chain with a 1/4" diameter bolt, with washers and a nut, to form a loop. now hook the winch into the loop, and pull out enough cable to reach the front eye on the ski. route the cable under the ski, but above the trailer frame. Getting the winch hook into the front eye requires an S-hook, or similar, but once you've got the cable hooked into the eye, make sure the ski is not secured to the trailer with any straps or binders, and start working the handle of the winch.
Watch closely as you winch it back, you'll be surprised how easily it slides back. You'd think, the cart would just raise up in the back where the winch is connected. I expected it, but it never happened. Crank it back until the ski is centered with the cart, depending on how full the gas tank is, stop and disconnect the winch, and hope the wheels on the cart have been greased recently, and the ground is smooth enough to roll the cart back from the trailer. I removed the 2" hard plastic castor wheels and put on 8" tires that air up on one trailer, then solid rubber wheels (harbor freight) on my other trailer. a little welding is all it took, plus, I put eye bolts down by the wheels, so I can strap the ski to the cart, because since no more castor wheels, I have to pick up one end to scoot it over as a way to steer. With out strapping it down, a 6-8' 2x4 is needed to lift & scoot the cart.