Ryan is correct -- although most of the larger sizes of the Toyo and Waterford Homers came with two top tubes, exceptions exist. I believe all of the larger (61.5, 64 cm) MIT Homers have the swooping second top tube -- although if the current generation is your goal a call to Riv would resolve any questions.
FWIW, I have a 67 cm single top tube Waterford-made Homer.
Tom, why do you ask? Are you pro or anti- two-tubers? How big are you? What sort of riding will you do?
Julian Westerhout
Bloomington, IL