Hi There,
I've just completed my build - Tigin Linn - Little House and Us - and I figured out a simple and effective method of carrying two bikes. If you have a look at these snaps it's fairly straight forward and it could be modified to work on a tongue box too.
I put an "extra" ball hitch on the tongue (drilled and screwed though one of the chassis members) and clamped a Thule bike rack onto it. Then mounted the bikes on that:

The hitch ball is 12" from the front of the teardrop and the coupling socket is another four feet away so when I am reversing the rear of the car does not get fouled up with the bikes.

Here is a link to the Thule Xpress 2-Bike .... it's really very good
https://www.amazon.co.uk/Thule-TH9700-X ... ay&sr=8-10Hopefully it's available in the US
Cheers,
Larry