Try this:
Connect the audio outputs [from the Tyros] to the aux inputs of the S750. This will give you the Tyros sound through the S750 speakers. They will be blended together, and regulated with the volume knob on the Tyros. If midid is connected, you can trigger everything from the controller keyboard.
It can get complicated with so many channels, and remember, the more sounds you stack together, the closer you get to a Farfisa organ sound! lol