Questions:
- Are these expensive bikes? A bike on a bike rack might as well have a sign on it saying "STEAL ME"
- Do you care if they are out in the elements? The bikes will get absolutely filthy on the back of your unit and components will corrode.
One of the main reasons we ended up with a TT was so that we had a fairly secure place to put our kinda expensive bikes. We put them in the bed of the truck which has a cap on it. We met some like-minded people with expensive bikes, and they put their bikes in a storage area with fork mounts on the floor. Some had as many as 4 bikes inside. It worked out well.
Since you don't have the option of putting the bikes in the truck bed, you'll have to make the choice of using a storage area (if the bikes fit), or putting the bikes out in the elements where they will get dirty and could get stolen. When stuff gets corroded and your bikes don't shift very well, you'll know why.
Good luck,
Dan