So- I have a SP-H9, and it is totally passive, so it should work… but maybe unpredictably. The EXP pedal is all voltage driven… so doubling the input voltage might throw off the range of the thing, you know? Like.. when you are on Y-mode…. you hit 100% halfway down the pedal? idk.
(side note for nick- that thing runs TRS because it actually includes a switch that will hit the AUX switch function on the h9)
Another thought…Let me experiment right quick-
OK it seems like I can fire up my H9 just fine with my SP unplugged.. and when I plug it in it reads the position the pedal is in. So thats fine.
Possible problem… when I UNPLUG the SP (which I imagine would be like switching) the H9 gets some junk-voltage.. and it had unpredictable effects on the setting. Like.. once it set it to 100%. Once it fired the Aux switch. Once it DIDNT. That would be a dealbreaker for me.
That might be the biggest problem with this plan. Midi would be better… and cheaper!
HOWEVER.. if you want a little more flexibility.. and you have a budget .. the DMC-3XL has an expression pedal in.. and would allow youto independently send MIDI EXP values to either or both H9s on a preset by preset basis… not to mention adding 2 aux switches, looper controls…and a whole dual-preset-control scheme. http://www.disasterareaamps.com/shop/dmc-3xl-gen3
But really- just connecting them via MIDI is probably cheapest and best.