Hi Cassp
1. Switch of keyboard split
2. Connect Midi out of 2nd keyboard to Midi in of 1st Keyboard
3. Set 1st keyboard to receive on channel 2 (Or whatever your arranger uses for the left hand split)
4. Set your 2nd keyboard to transmit on channel 2 (Or whatever your 1st keyboard requires for the left hand split)
5. You now have a 2 manual instrument
If you’re not sure about something, (And this may sound Alien) have a good read of the owner’s manual.
If you have a Tyros 2 the Reggi Stick has all the set-up done for you, so you just add your second keyboard.
Hope this helps

Bill