Amelio,
I have never tried to control the Tyros via another keyboard, but it shouldn't be difficult: all you have to do is to match the Tyros "midi in" channels (left and right) with the "midi out" of the other keyboard. You could also split the other keyboard to control the Tyros via left hand and be free to play at will with your right hand.
Can you imagine having at your disposal a 76 or even 88 notes Tyros?
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Korg Kronos 61 and PA3X-Pro76, Roland G-70, BK7-m and Integra 7, Casio PX-5S, Fender Stratocaster with Fralin pickups, Fender Stratocaster with Kinman pickups, vintage Gibson SG standard.