One thing to think about in the above pictures and the double receiver you are thinking about, is in the above pictures they are using a different hitch than most tow dollies use (receiver attachment, common on 4 down hitches, verses a tow ball on a receiver). I was thinking the same as you. physically I think it would work but the upper mount, if the bike rack extends out pretty far, might make it impossible to squeeze the coupler of the dolly over the tow ball, unless you hitch up first then put the bike rack in, or use a drop receiver into the dual receiver.
Might not work for your set up but on my motorhome the receiver bracing is fairly exposed and I bolted onto it a lightweight receiver like the ones they sell for trailers that have the 4" square bumper, but mine was built to be bolted under a truck bumper, but with some different bolts I bolted it to the square tube cross brace, so I now have the original receiver, and now a second receiver about three inches higher and about a foot to the left of center, far enough that the hitch could never hit it unless I was backing and jackknifed it(impossible with a dolly). I use that second lightweight receiver for my bike rack. It being off center only helps it be out of the way of the ladder anyway.
Hope you find something that works for you.
