But.................
Be very careful that the dry cleaning bag does not have some sort of printed message/logo, etc. on it. If your carry-on winds up getting separated from you (you wind up being forced to check it for some reason or another) and if the bag winds up sitting in the sun for just a bit or being placed in or next to a warm engine part within the airplane, the logo can melt off the plastic bag, either on to your dress or a neighboring item within your bag. I know from experience....not a pleasant surprise!
I would personally wrap the dress(es) in plain white tissue paper, which would also provide a bit more cushion within the carry-on.
If you have a long, light garment bag (similar to something you might purchase at The Container Store), you could put your dress into that and then fold that within your bag....maybe layering a few sheets of tissue paper within the garment bag before you fold it. Then once you land, you could take that garment bag out of your carry-on and just carry it separately (with either just one fold to keep it from dragging) or just hold your hand over your shoulder as you hold the bag against your back (I hope that makes sense)....that way it would reduce the time scrunched up within your carry on and will allow more time for the wrinkles to fall out.
Sometimes I have even used skirt hangers (the light plastic ones ones) to clip the shoulders of a dress (cushion with some tissue paper) vs. using a traditional hanger that might make point marks in the sleeves. And I always use baggie type ties to hold hangers together, so one does not go rogue and slip down to damage another garment.
And, if you are really lucky with a very nice flight attendant, maybe you could remove the two dresses from your bag (assuming with some sort of protective bag) and have them hung up during the flight (works better if in business class or at least the upgraded coach section), but may even work in basic coach if you ask very kindly and luck up with a very kind attendant.