Set the sleds together in the garage nose-to-tail as tight as you can get them, and measure. If you have the exact measurements of the inside of the trailer, simply drawn out a rectangle on the floor with tape.
To get down to 6 feet it is going to be tough even if you offset them as much as you can for the 12 feet. Getting them in the trailer is going to be a pain even if you have reverse. Most cargo trailers do not have a front door, so you will have to drive one on, then back (or wrestle) the other one on. Then you will have to drive one off, and then back (or wrestle) the other one off.
Watch out for door width, that can complicate matters a bit.