If your starter is like the one in my truck (mechanicaly activated by foot) then there is only 1 cable connection and I would advise not to use James' method, since there's no "after the solenoid" connection.
It sounds like a good method if you have a starter solenoid, but I would still keep the polarity the same just to play it safe.
That said, I've jump started many 12 volt machines using a 24 volt jumper pack. I connect the Hot lead to the battery or starter post. Then I try to have an assistant crank the engine. Once the assistant turns the key to the start position, I touch the gound lead (away from the battery because it'll spark). You get a surge of voltage, engine spins rapidly and usually fires right up. As soon as it fires, remove the gound lead. This method has worked sucessfully for me on many occasions. It gives a momentary shot of higher voltage for the starter, but usually the rest of the system isn't effected.
And again I'd advise to keep the polarity the same and don't let the vehicles touch.
Merle