I am looking for a way to switch sounds and articulations in both Omnisphere and Trilian. Does anyone know any footswitch that would be a perfect match for those programs? Maybe someone knows what kind of pedals or switches Eric Persing uses when he demoes Trilian to George Duke and Greg Phillinganes?

I use a programmable USB foot switch. Had it for over 10 years. There are other brands but the X keys has proven itself to be the best quality for me. I also bought a basic foot switch for $12 and it  malfunctioned after a few months on one swicth. 
They can be assigned any combination of keystrokes. I use the cheap one to control Sonar with R  W and Space. But I now lost the Space. Here's a review I wrote on GS. 
 
https://www.gearslutz.com/board/reviews/923689-p-i-engineering-x-keys.html

 
 
 
 

Now that looks like a solid solution! Thank you for the reply!
I'll have a look at their page.
Do you know if there would be an issue installing two pedals?

 
Here a three-switch midi pedal :  Logidy UMI3 on amazon.com  80$US.
 
"The UMI3 is a USB foot controller which shows up as a class MIDI device on Mac or PC. No driver installation is required. It is powered through USB. Each of its three switches can be configured to send any type of short MIDI message (note on/off, continuous controller, program changes...) An external expression pedal can be connected to the "exp" input which can be configured to send any continuous controller message. A status LED indicates power and USB activity. The enclosure is made out of steel."  from logidy.com.
 

Now That's cool too. But what I need is one that goes directly to midi devices like my Lexicon reverb to change patches. So I guess an old school Midi foot controller. 
That swicth would work if you have your audio interface that has midi ports hooked up. 
 
Oystein, Go to the p i Engineering web site  http://xkeys.com/
and they have all the documentation.  I'm certain that each pedal can be programmed individually and assignments are stored in the pedal now. Under XP this was not so.  I see they now support Android so the company is progressive. 
If you need more than 3 switches they also make these too. Looks like with this device you could make your own switchboard out of any contact switch.  
http://xkeys.com/xkeys/xkswi.php
 

Any switch can trigger an action from your computer. Connect up to 12
switches to the X-keys USB 12 Switch Interface and program each switch
with a function. Any keyboard shortcut your software uses can have a
designated switch (copy, paste, save, open, close). Switches are easily set
up to launch applications, open files, or type short lines of text like names,
addresses, and account numbers. Switches are connected via Assistive
Technology standard 3.5mm (1/8”) phone plugs.
 
 
 
 

Wow! Thank you for good answers here! I have had a brief look at the x-keys site, but I will also check out the UMI3. It's just what I've been looking for. I just wish that some local dealer would have it. Importing anything above $25 adds all sorts of extra taxes on the item here in Norway. Even if it's not possible to buy it here. Thanx again!