Cut out the keyboard area of the second board, and cut some gaps for the switch, so we need to make sure the gap is the same the switch on the width.

Note: To make sure your Keytar can work after complete, you should make sure the switch position is right under each keys (that is the position should be at the center of the key)

Then fix the switches with glue gun.

And paste two boards together with glue gun or double-sided tape.