My tandem is custom, but it is fairly standard sized. We used a kidback on the back when Em was smaller and have moved to crank shorteners. (All of these items are explained with pictures on the link below).

My tandem weighs the same as any other road tandem - probably in the mid - 30's? I'm really not sure - but no where near 50 pounds. With the kidback it was heavier, but now it is not. It handles great - much easier to ride than the trailer bike.

This is a great place to start:

http://www.sheldonbrown.com/tandkids.html

Here are a couple of other photos of the tandem in action: