MPI still sells the cable for this application. The cable is used for charging the 4-stroke battery, NOT jump starting it.
You hook the cable to the 2-stroke headlight plug and hook the other end to the 4-stroke cables. Let the 2-stroke sled run for awhile. Disconnect one end of the cables and try the 4-stroke. If it isn't enough repeat the first steps.
Don't try the start the 4-stroke with the cable still connected. It could fry the electricals on the 2-stroke.
I have the cables in my pack all the time. I have only had to use them once when it was -40 degrees in the morning. My sled fired once and died. I didn't have enough juice to fire again. Hooked up the cables to the sled next to me and in a minute it was started.