Saotome Sensei did not learn two sword from O-Sensei or any other teacher. It was something he was interested in and asked the Founder about. O-Sensei told him that if he was interested in two sword work he should research it himself. While I am sure that Sensei was aware of the various movie depictions of two sword and may have seen two sword done at various embus in public demonstrations, his two sword forms are really his own development. While he did experiment with two sword while he was still in Japan (according to my understanding) he didn't create what is now the full repertoire, which is quite extensive until the 1980's. I have over 25 forms written down from video taken back in the 90's. While I think these cover the main principles and movements, there are many more variations of these forms on that video. At the first Aiki Expo i was invited to do a demo and Sensei asked me to do two sword as part of what I did. I distilled the forms down to a set of twelve that I thought were representative of the most important principles. These are the forms my students perform on their San Dan exams although that is just my dojo and not an organizational requirement. I do not believe they are widely known or practiced outside of the dojos of a few direct students.
Unlike regular sword or staff work in which the hands largely move together, two sword is a bit more like we use our hands in empty hand. They can be used separately, offensive and defense are simultaneous but can be done separately. Anyone familiar with Saotome Sensei's empty hand can easily see the connection with his two sword work. In fact, at the Aiki Expo demo, I did ten two sword forms and then did the equivalent empty hand techniques.
Like most practice, Sensei's two sword goes through a progression. Initially the forms are done in a way that emphasizes good structure and proper geometry. A fair amount of bokken to bokken contact is made. Later on, the same movements can be done with increasingly less contact focusing more on changes in timing and spacing. There are even a few forms in which one initiates an attack and then executes a technique off the response.
Here's a video clip of two of my seniors practicing for a San Dan test. It's a less formal presentation than the clip Katherine posted which was from an actual test. Two Sword Practice