With such a large range of character sizes you might think it would be impossible. I don't think so.
There are multiple ways you could make it work.
1. Characters of similar size can only share a skyscale mount. So don't allow asura and norns to share
2. Allow a tonic to be used that makes two people similar height and also keeps the aesthetic. Those with the tonic activated can ride a two seater. Or, when a person mounts the two seater it transforms them into an aesthetically pleasing and proportionally acceptable form.
3. Use a mount skin which has a compartment that hides the characters or second character and avoid the whole proportion thing entirely.
4. It just occurred to me the easiest way most likely would be to have the two seater mount owner turn into the skyscale and the other person ride it!! THIS WOULD BE AWESOME!!!!!!!!
I am willing to pay the price of a whole xpac just for this feature.