I just did this when transfering two PWC from single to a double trailer.
I just tied my ski to a tree in the yard using the rear mounting brackets, then I drove the trailer away. The rear of the PWC will be on the ground, and you will only need to slowly lower the front. I did that by using the winch strap.
It was little tougher getting the PWC back up on the new double, as I had to tilt the trailer and then winch it up. Tough when I had the 2nd ski already loaded, but it all worked out ok