Samurai used ONE Katana, since it was a two handed weapon. They had more swords on their belt, but only used one of these at once. Since the Katana was a soft but thick and heavy sword. (it never broke but was easy to bent)
If you see pictures of a Ninja or Samurai with two Katanas it is fiction. Ninjas rarely used Katanas at all.
EDIT: deleted the first line, it was already adressed
The katana is neither light nor heavy. It's actually pretty average in terms of weight. Unless of course european longswords were just as light, which would make me believe that dual weilding them would be EASY (sorry but it is NOT heavy to swing. AT ALL. Only people who never held a sword in their life would tell you otherwise)
And ninjas are still being debated on whether they existed or not, but on thing is for sure: if they DID exist, they used whatever they could find. If they COULD kill a samurai they could take his sword and use it without much problem, but they wouldn't use it for long (the katana might have a super sharp edge, it became dull rather quickly, and maintaining it was rather difficult for someone untrained in doing so)