Personally, I think the dual hitch is your best option. I know it will make it a pain to pop the rear door when it's on, but that seems better than the tongue, IMO. my concerns with the tongue lengthening are that it will drastically increase tongue weight. Your van maxes out at 400 lbs. Putting 100 lbs of bikes, plus the weight of the steel, combined with the fact the extra length will push some of the weight already on the trailer onto the tongue, you'll easily go over that. Plus this will affect your ability to sell it down the road. Many buyers (myself included) would be scared away by a homemade structural modification, regardless of how well it appears to be done.