I have been looking for solutions to carry a couple of bikes inside. Looked at toy haulers and travel trailers. There are a few lower profile, lighter toy haulers, but they still have the ramp back door.
I looked at several bunkhouse travel trailer floorplans, some with the outside storage door. Usually the outside storage door wasn't large enough to fit the bike with wheels on.
I recently came across the Timber Ridge 270 DBHS (31'8" long). A few things I like about this for carrying bikes, although it may not have the capacity you are looking for:
1. Bunks across back rather than side
2. The above allows a full-sized door on the street side of the trailer to access directly to the bunk/storage area. With this direct access full-sized door could your tandem go all the way across and fit?
3. The "storage door" is larger than most I have seen (although still smaller than the full-sized door).
4. Very good quality trailer and nice interior (but also a bit pricier and heavier)
Curious to see what else you consider. Good luck!