Martin,
There are a few ways one might consider:
1 - If your SX700 or Tyros 4 has the ability to apply the controller volume to an aux input, you could route the audio of one keyboard through the other, and control it that way. I just have an S950, and it can apply the controller volume to the mic input but not to the aux input, so in my situation I'd be sacrificing stereo to get this to work this way.
2 - Find a dual output controller pedal (note: not a stereo volume pedal). Note that the outputs need to be completely independent outputs.
3 - If you're not using the onboard speakers, and have access to your mixed audio signal before it goes to your power amp / speakers, you can get a stereo volume pedal (not a controller pedal), like what a guitarist might use, that you can insert between your keyboard submix and your final mix or your speakers/amp.
4 - Do not try: connecting one pedal to both keyboards using a Y-cable. In order to "read" the foot pedal, the keyboard has to apply a small voltage (such as 5 volts) to it, and if the voltages used by the two keyboards happen to be different, or if in other ways the circuits are incompatible, you could damage the foot pedal inputs on one or both keyboards.
- Greg