http://www.flickr.com/photos/67610004@N03/6978983313/in/photostreamI simply secure the seat post to the front vertical post of the rack,using toe straps & tie wraps or whatever as you would when using a tandem.
The front wheel rests in the frontwheel cradle & has a toe-strap to hold it insitu.
The rear wheel is secured,via a toe-strap & tie wrap to the horizontal spine of the tandem rack.
Simples.
The rear vertical post of the tandem rack is not used for the Langster or Trek but is used for the Longstaff which has slighter longer chain stays.The rear wheel is secured to the rear vertical post of the rack using a toe-strap and/or tiewrap.
I use a toe-strap around the front wheel & down tube on all bikes being lifted onto the roof to stop the front wheel swinging about during the lift.
hth.