The FR stock rack (or equivalent) ProRac Proformance Bike Rack
would probably work with your tandem if you took great care to safeguard the bubble window when loading the long bike.
One of the considerations in putting bikes on A Frames is the tongue weight. The trailers already have a TW in excess of 400 lbs, and a few bikes on the tongue could overload the HW on some vehicles. I would look into weight distributing hitch to help balance the load.
AFrames are so tongue heavy that from a balance perspective it would likely be safe to put one bike on the back and maybe two. The problem is the lack of anything to support them without a significant amount of custom welding that would void your warranty. The stock piece of thin sheet metal that is laughingly called a "bumper" is too weak to stand on.