I wouldn't do it for two reasons. First, both keyboards do not use the same voltage power supply--one is 12 volts and the other, if I racall, runs on either 16 or 18 volts. That could cause a problem, depending on where in the output circuit the volume control is used.

The second is you may have an impedience mismatch, again, depending on where in the circuitry the volume is regulated. One may be on the output, while the other may be in the pre-amp section. Not a good idea to hook them together.

The best way to accomplish this safely is to physically attach a pair of foot volume controls together so when you depress one, the other also is depressed at the same time. Essentially, they would be side by side, the bases glued together and the pedals attached together by means of a lightweight bar or screws.

Good Luck,

Gary