OT: Footswitch

Guitarists use routing switchers (like this) to change amplifiers and signal chains on the fly. Fancier ones are basically foot-operated patchbays with unlimited possibilities. But these may be much more functionality than you want.

Here's one that contains two switches, either or both of which can be configured as momentary-on.

Personally, I'd probably prefer an expression pedal for the doubled synth so that a) the transition wouldn't be too abrupt, and b) I could adjust the balance on the fly.

You can use any kind of foot controlled A/B switch to do that, though all the ones I've seen were made for guitars and are Mono. The switch has two outputs. Connect one to the PA. Click the switch and you hear the synth through the PA. Click the switch again and you don't (because that output isn't connected to anything). Here's an example of this kind of latching switch, but there are many models to choose from. This is, basically, the same kind of latching device bitflipper posted a link to, and is the cheapest solution. He also posted a link to one that has a momentary switch function and will do exactly what you want. http://www.zzounds.com/item--BEHAB100

This would work, I think http://www.amazon.com/Ern...dal-Mono/dp/B0002GZ052

You're on the money there, Bit. My bad that I didn't say up front that the synth I want to control is the Yamaha VL70, a box and patches developed specifically to be used with a wind controller (Ewi, Wx5, etc). In part, this means the "expression" you get comes from the wind controller, so there's no expression jack on the synth. Also, this is for live performance, so stereo is not a big issue. Perhaps I should go back to the momentary switch idea, set the VL's volume on the PA and go that route.
The big impediment now is that, having re-read the websites, I found both units ONLY run on batteries, not good for live perf. Darn. I'm flipflopping like a cheap politician, but I guess it's just the process of sussing out the pros and cons of a plan. Thanks again to all for the input.

What about tone degradation with the Morely et. al.?

I have an Ernie Ball stereo Volume pedal, It's passive and will work with Keyboards. This one shown has a pan option too. Mine is 20 years old and has never glitched.
 A keyboard player I worked with used 2 of them. One from his Piano and the other from a sound module he used for Organs and Synths. He had a little sub mixer and even ran his vocal mike though it.  He was always on the money in the mix. He could feed you stereo or mono via XLR too. He used the mixers aux to a powered speaker for a monitor. Excellent set up.

Late to the party here, but I'll add some options for those with similar problems.  MIDI Solutions boxes insert inline with your MIDI device, and generally derive their power (with few exceptions) from the MIDI stream itself.  I use the Event Processor Plus as a programmable toolkit for all kinds of problematic MIDI routings.
 
Simpler solutions (with easier programming) for the narrower applications found this thread might be the Pedal Controller, or the Footswitch Controller.  These boxes are similar to the discontinued Anatek line (specifically here: the Pocket Pedal), but have more versatility built in.  And the programmability doesn't involve setting DIP switches.