I finally had a picture taken this weekend, so I thought I'd post it and tell a little about my experience with creating this custom configuration. First off let me say, this is not the cheapest and may not be the safest way to transport your children. Trailers can be had for around $150, which are low to the ground and some even say the trailer won't tip over if the bike does. If you're looking for low cost, and are not an experienced cyclist stop reading now, and go do something better with your time. I personally had several reasons for opting for this method, and I decided that I would ride only on greenway trails or similar to ease my safety concerns (well, and a little bit on some safe roads to get to the trails).
One of the biggest reasons I decided to create the dual-child seat was because I wanted to have an interactive experience with my boys rather than cart around a mobile playpen of sleeping children. With this setup I can actually give the boy on back a water bottle while riding, they can hear me talking to them, see the animals, or trucks or bridges I point out to them. The two boys can't seem to leave each other alone when in a 12'x20' play room so I can't imagine what they would do to each other left in a bike trailer for five minutes. While buckled into their child seat neither can reach the other, so there's very little fussing. On the bike they get the feel of riding, and have (almost except for my backside) 360 degrees of vision to look around. With all the trail riding we do there are lots of curb cuts and vehicle deterent posts to manuver for which we slow down but don't have to stop.
The largest negative to this set-up is the top heavy nature it brings to the bike. When stopping or starting it is critical to keep the bike perpendicular to the ground. This is basically going to feel like a loaded tandem without having a stoker to help you out. If your youngster can not keep themself from flailing around then they don't really belong on any childseat let alone this one. When I was still waiting for the "Rock Steady Kickstand" (see ingredients below), I the bike got out of perpendicular enough that it went down (gently) while loading up in a gravel parking lot (no one was hurt). This was enough of an example that whenever the oldest boy (in the back) starts moving around I can reason with him (remind him) and get him to settle down.
With all that said this has really been a wonderful vehicle we all enjoy thoroughly. I can get some exercise, my wife can get caught up with her work and the boys can get out of the house. There are all sorts of things to see and do along our 15+ mile trail that each time is like a little adventure. Last Saturday we did our first ~30-35 mile ride, and were gone for 6 hours, stopping along the way for lunch at Souper Salad, the County Stadium for a Marching Band Competition, and at several parks and playgrounds. We've also been to fairs and festivals and museums on our route and we go right by our local minor league ball park, so next year the youngest will be old enough that we'll stop there as well. I don't mean to bore anyone with the details especially since your results would surely vary from these; I mention them only because I really didn't expect anything like this when we began, and now there are so many things we get excited about looking for along the way. I don't think this setup was critical for us to take these rides, only for the enthusiasm it instills in it's riders which leads us to ride more often and longer than we might have otherwise.
Here's a list of the equipment I used to create my setup. (I don't recommend this to anyone.)
Xtracycle Kit $250 + shipping (now costs $400)
Custom Built Rear Wheel $120 @ REI (Rhinolyte rims, 36 14 ga spokes, on a Deore hub)
Peapod (bobike) Child Seat $70 (inc shipping) new on eBay (don't forget the MTB adapter kit ~$20)
Old Childseat (w/ bolts) $10 @ a garage sale, removed bolts from steel frame, cut a bit to fit
Rocksteady Kickstand $100 (also available on the xtracycle website - critical for loading)
Deore V-brakes & levers $45 (may as well do the front though not required like rear)
Helmets for the Boys $70 don't get the infant helmet, get the adj. child if at all possible
Season to taste (items not required, but some are pretty useful):
Specialized Body Geometry Seat $20 (on LBS clearance table because someone replaced on new bike)
Light & Motion HID Light $350 from Performance Cycle (local, no shipping required for me)
Cateye LD-1000 $30 (I've now seen it for $25)
Safety Flag & Vest $20 (I like Illuminite!)
Handlebar bottle mounts $15 (don't compromise your balance by leaning over all the time)
Wider Handle Bars $15 (just a preference, I feel more secure with a wider grip)
Rear View Mirror $15 after I broke the bar mounted, I got the helmet (helps see the kids)