Are you sure you have your dimensions the correct way round? Every large backdrop I have had experience with has been either 2.5m or 3m wide by five or six meters long, the 2.5×6m would be pretty close to your 9'×20'. In these big backdrops the stitched over hanging loop for the pole is always stitched on the shorter edge. To be honest it doesn't surprise me, since given the weight of fabric involved, running a single support pole longer than about 3m (13') seems like a recipe for equipment failure.
Actually I don't think you are too likely to find a single length of cloth that will be wider than 3m/13' since the width is going to be limited by the loom, and the ability to handle the roll of finished cloth, there just aren't very many uses for a single piece of cloth that is larger than that. Without demand from another source it wouldn't be viable to manufacture for the very small number of photographers who might want an oversize backdrop.
Also a single 3×6m cloth backdrop is quite hard to work with, trying to keep the wrinkles out is a bit of a nightmare, I know I have a white one for large product photography. Oh and I don't know about US washing machines, but here in the UK that single 3×6m cloth is about the maximum load size of our average washing machine. So keeping it clean might be an issue. Ideally once washed and ironed you would want to put it on a core to roll it, just to keep it from creasing. Personally I think that you will be better off using two drops, just overlap the two supports and bring one over the top of the other.
Actually at the moment I am looking at replacing my white muslin with a roll of PVC instead, since I need to fix the cloth on a roll anyway, so ease of transport will be the same. I have found 2.5×5m and 3×6m white PVC fixed to a core for around £100 to £120 on Amazon, which seems like a really good price. It has the distinct advantage that it doesn't crease like the muslin does, very useful for product shots, and is easy to keep clean, just give it a wipe over. It seems to be recommended for "Cake Smash" photography because of the ease of cleaning. Two rolls with a 150mm/6" overlap should make for a very good seamless background. One other advantage of PVC is it is waterproof, if you are using it outside.