El Arbol will not be the first tandem tall bike. Check out this lamplighting bike from Chicago at the turn of the 20th century. Read the Full story here:
Or how about this family contraption, complete with treddle-powered sewing machine:
Thanks Flickr friend Whymcycles
More recent but also notable, from Chicago Critical Mass, via Georgeaye. The photo has been viewed 4500 times on Flickr.