It might at first sound a little odd, but a synth players best friend can be the guitarist. How? Have you seen the average guitarist's pedalboard? There are a lot of cool effects out there for guitars, and they can add a completely new dimension to a synth. Synth players cover a wide variety of duties, from driving pianos and bass lines, to foundational pads, atmospheric soundscapes, and doubling leads.
While synths are powerful on their own, there can be something fresher, and more organic with a guitar effect pedal. Get a dual oscillator going with one sine and one saw tooth – then run that through a tube screamer or RAT. If you are going for that Rudess/Petrucci thing, this could help glue the two instruments together. Or, take a pad and run it through a flanger or phaser (especially if the guitarist is doing the same thing), and layer that under a clean guitar. Also try experimenting with the flanger/phaser on the synth being slower (or faster) – but in relative time to the guitar, to create even more of a contrasting texture. Borrowing a tremolo pedal to apply to a pad, or strings, that is layering the foundational chord of a guitar progression, can also add a nice new feel. Something that gives a similar sense of a delay on the guitar, but much more interesting. If things require a bit more complexity, a synth player can always use the same multi-fx processor as the guitarist. That way you can share presents and layouts, that will make things a lot easier – and more fun – both in the studio, and on the stage.
A good example is, let's take that Rudess/Petrucci synth/guitar layering approach. Take one of the FM sounds from the Yamaha Montage (those have a sound familiar to a guitar, but with a more percussive – and slightly funky sound). Now run that through the ProCo FAT Rat with the MOSFET clipping mode. Keep the overdrive at the point where you get rich distorted harmonics, but it is still intelligible – then sweeten it with the tone knob. Sit that up under the guitar solo, or if playing a melody – pan the guitar and synth out (maybe 30-50% L and R – respectively), and you have a whole new vibe to the song that really allows the instruments to gel together. Simple things like that can really elevate a sound/song.
These are just a few ideas of how the tools of a guitarist can enhance the performance of a synth player. But, don't just stop here..... Dare to experiment with your own ideas.