Agree with Don, especially the part about moving them around.

I did this for a while using an A-frame stand. Firstly, it was hard to get the spacing and angle of the boards just right to be able to reach the keys and access all of the controls. It was fun but because I didn't want my living room to look like a music studio, I regressed to a Tyros + BK-7m beside it.

Some reasons to do this might be:
(1) to have a 3rd RH melody voice readily accessible for occasional use. These are both 61 key boards; a 76 would solve this problem.
(2) You could configure one keyboard to play chords in any inversion, and the other with an "on bass" chord recognition. This might conserve hand movement when playing consecutive non-slash chords. Of course you'll have to move your hand back-and-forth between the keyboards when it's time to play a slash chord. Also it's tricky to send the output of one arranger to the other's style engine. You might need a special user style consisting of a sustained CM7 with root bass to do this reliably. You'll also have to designate which channels receive chord and bass data in the MIDI setup.
(3) Faux "chord sequencer" or "riff mode": You can have MIDI sequences staged on one board which contain common chord progressions. You can even assign them to multipads. When you trigger them, they send the appropriate notes to the chord recognition engine on the other arranger so you can jam away with both hands while the board goes through the progression. You'll still have access to variations, fills, and it's transposable to any style or key.

Note that you can also do this with only one arranger by looping the multipad MIDI out back into the MIDI in. It's tricky but I've done it. This feature was native to the PSR-8000. I wonder why Yamaha dropped it?
Have fun! These are amazing machines and we all use them differently- maybe even differently than their designers imagined!!