Hi Chris,
Bravo to you and Mr.Dekker! This is a very good collaboration and performance of "Legends". You both have a wonderful and sympathetic sense of timing which aids coordination in the playing. One session or not, this performance has much to commend it. For example, it is sometimes difficult enough in solo piano pieces to manage dynamics and shadings as intended, but doing so as duo pianists must be even more difficult.
Regarding recording, for duo piano works I would think that a matched pair of small diaphragm omni-directional mics would be the best choice as they pick up the room ambiance as well as the sound as it leaves the piano. For this application, cartioid mics might be too focused on the piano per se. I believe that would give you even more a blended and finished sound as captured by the recorder.
Again, wonderful playing! Looking forward to the second half.
David