The Xtracycle cargo bike is famous for hauling everyone and anything. Occasionally you also need to haul an extra bike. But normally carrying a bike on a bike is typically somewhat awkward even for a longtail cargo bike. You can see some other nice solutions on the Xtracyle gallery here: http://www.xtracyclegallery.com/2010/10/519-mike-xtracycle.html or http://www.flickr.com/photos/grrsh/4438906597/.
Here is a simple solution using a commercial fork mount carrier adapted to fit the side tubing of the Xtracyle.
Parts list -
1. Fork Mount Bike Hitch available through Amazon
or many others including Niagra http://www.niagaracycle.com/product_info.php?products_id=801159
Quantity 1 - price ~ $15
2. Piece of ~12 inch by 7/8 inch (22.2 mm) exterior diameter pipe
Quantity 1 Home depot, Loews scrap or $5
3. Bolts 1.5 inch long, 5 mm or larger
Quatntiy 3 Home depot, Loews scrap or $2.00
Step 1: Step 1 - Drill and Bolt
2. Bolt the fork hitch carrier to the pipe.
3. Drill a 3rd hole to mount the pipe to the Xtracycle front tube
Step 2: Step 2 - Mount the Bike Carrier to the Xtracycle
2. Bolt the bike carrier pipe hitch to the Xtracycle tube.
3. Remove front wheel from bike to be carried.
4. Place fork in carrier.
5. Tighten quick release bolt.
6. Test for stability and ride.