Do you have to carry them on the rear of your TT ?
Personally, I think you would also have to add shocks as well as a rear fabrication.
I think TT's bounce around too much for a rear rack, and I don't like not being to see them while they are bouncing.
Carrying them at a location where you can monitor what they doing may be a big plus if one of them decides to go off on it's own without you.
If the TT mfg wanted people to carry bikes back there they would offer the option. Instead, they leave it up to you. Remember, liability, this is a mod that YOU made and relieves the mfg. of any liability.
What about using other options ? Where you can see them.
Truck bed
Trailer Tongue mount bike rack
Front hitch on truck