I think making a harpoon realistic (reads: newtonian) is probably less fun.
Supposedly the harpoon just requires a lot more finesse than people expect.
As I understand it, it accelerates you towards each other. You just need to output more force then the other guy, then somehow not hit the object yourself.
(I think it's a good solution to a 1D problem, but unless you are going to slam into the wall yourself, or nearly anyway, it's to easy to 'orbit' around the obstacle).
Tentative suggestion: Kill their engines first?