why not sword arm and and a sword in left hand too? Some of the horse rapists were duel wielding those retarded looking scimitar swords they use so I imagine Jaime being a master swordsman could as well.
Because Jaime can't ride as well as the dothraki who live in a saddle, also because dual wielding on horseback is a stupid idea regardless, and because arm-sword would be fucking useless when you have no wrist movements, I mean come on mate.
You should ask why didn't Jaime attach a giant dildo to his arm for the affairs with Cercei, while you're at it.
And btw, what exactly is retarded about the dothraki swords?