Andy,
I would just go with a quality switch that has a 10A rating or better. I have seen these in some of the better hardware stores, and in some home improvment stores. You will need a double pole double throw (DPDT) and some places will list them as 220-240V center off switch.
I do run the pre/post EGT's and I did go with two gauges. I wanted to be able to see the differences simultaneously, I'm just nosey. But I do run the same set-up you want on some of my other gages, it works great.
Here is what I did and maybe it will help you.
The switch just needs to be of good quality, there isn't to many styles to choise from, so it will probally be a "bat" switch with a chrome handle.
You will have two wires coming from each probe, don't cut the wires, if there is exsess just ty-wrap it up. You want the wires to be the same lenght, so the readings are valide (wires have a certain resistance value per foot, so if you trim the wires and they are different, the readings will be off). These wire coming from the probes should attach to the top and bottom terminal on the switch (observe polarity). The center terminals on the switch will go to the gauge, again watch polarity, and make the wires (use good quality copper wire 16-14 guage) are the same lengths as each other. Thats realy about all there is to it. I use this set-up to share a pressure and temperature gauge. Since Autometer uses a one wire sensor, I can monitor 4 subjects with two gauges. When my switch is down, I am monitoring engine oil pressure and temperature. When the switch is fliped up, I am monitoring Transmission TC pressure and Rear-end temp. Good luck and I hope this helped.
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