Hi Roger,
When I first started playing keyboards, I wanted to link the KN650 to the Yamaha 500 keyboard in order to use some of the pianos, strings and the rock-n-roll styles. In those days Midi was very new (and expensive) and not much was known about it so, to help me, my Dealer recorded the sounds and rhythms I wanted from the Yamaha and saved all to a floppy disk. This was placed in the floppy drive of the KN650 (Master keyboard. Whenever I wanted to use the Yamaha sounds etc. all I did was turn down the volume to '0' on the KN650 and vice versa. Of course if I was using both keyboard sounds, the two volumes had to be adjusted to reflect whichever keyboard was to be the 'Master' for that particular tune. I have to say it worked very well for me and I used both keyboards this way for a very long time.
I know you can use a USB stick with the Tyros 4 but I don't know if the Casio uses it. If not, you can get an inexpensive portable Floppy Disk Drive which has a USB plug on it. I regularly use it with the Tyros 1 at our Club and it works well.
Hope this helps in some way.
Audrey