Can I use one of these switches to switch 2 different voltages ?? : https://www.ebay.com/itm/303173473870

 What I'm thinking of doing is using one pole to run the normal 120v lighting in my house and the other pole for running 12v emergency lighting for use during a power failure  .   I wouldn't see how this could be an issue as the 12v system would only be active when the line voltage is dead .

What would happen when the line voltage came back unexpectedly though?, I’m on the understanding that backup systems need change over switches?

It's a double-pole switch and not a 4-way switch, so as long as you are switching voltages and currents within it's rating, then it should be fine.

 @ Angry Horse : The 12v portion will be controlled via a relay with a 120v coil . Since the switches will be double pole , 12v will be running thru 1 pole and 120v thru the other .

 @ HIDLad001 :  I just bought a couple of these switches . I'm gonna do some tests when they arrive . Personally , I don't see how it could be a problem as these switches are basically 2 separate switches inside one housing , controlled by one lever .

The problem I see is that if you're going to be using one switch for the mains voltage and battery back up is that both lights would be on or off at the same time. 
Unless you're using a mains contactor to hold the battery supply in an off position when energised.

I think, at least from what he told me in a another thread, was that he wanted to have a fixture with multiple lampholders and have one dedicated for 12v operation. I'm guessing the relay is like what they have in emergency lights here, it being a simple relay used to keep the battery backup supply off until a loss of AC power is detected.

@ Angry Horse : The 12v portion will be controlled via a relay with a 120v coil . Since the switches will be double pole , 12v will be running thru 1 pole and 120v thru the other .

 @ HIDLad001 :  I just bought a couple of these switches . I'm gonna do some tests when they arrive . Personally , I don't see how it could be a problem as these switches are basically 2 separate switches inside one housing , controlled by one lever .

Yeah that makes sense, I don’t have a back up system at home as we very rarely get power outages in Cheshire, not even in storms!, but I believe there’s very strict rules about it in the UK?
If you’re generating 230 volts from a back up generator, and that find’s it’s way down the service lines during a power cut, then you could easily kill line men that are working on equipment unbeknown to yourself at the time, so a change over switch is needed?

 @ Sammi  :  Yes , there will be a relay inline with the 12v circuit do disable it when the power is on .
 
 @ joseph :  Correct .

 @ AngryHorse :  That rule applies here also ! If you're running a back-up generator , you MUST have either a transfer switch installed or shut off the main breaker to prevent back feeding the utility lines and potentially killing someone !  95% of my system will be 12v used for lighting and a few USB outlets for phone charging .  I might use a generator for powering the well pump , fridge and furnace . I'll cross that bridge when i get to it ..

@ Angry Horse : The 12v portion will be controlled via a relay with a 120v coil . Since the switches will be double pole , 12v will be running thru 1 pole and 120v thru the other .

This is not acceptable unless we are talking about a special switch explicitely rated for such use.
The problem is in the insufficient insulation between the mains and the 12V circuits. Normal switches are designed to provide just a "working" insulation,  not safety "double" insulation between the contacts. So they can not guarantee they would not make the 12V circuit energized at mains potential and so electrocute someone (because he would be assuming it is a safe low voltage).

The way to go with using just common parts is to have that switch on one circuit only and use a relay to switch the other ones. Relais with rated double insulation between the control coil vs the load contacts are readily available parts, multipole switches with rated double insulationbetween poles aren't.

Note it isn't by far just about the dielectric strength, on the contrary the main difference is in the way how the thing is constructed, whether there are sufficient barriers sepaating those circuits even when the switch fais.
E.g. in normal switches both poles are in one common cavity, so when some metal part like contact pawl breaks lose inside, it may short out poles together, causing the 12V becoming exposed at mains potential.
When used as intended (to break both leads of the same circuit), such event would justcause a shortcircuit and trip the fuse/breaker. But here it will make the whole 12V part "flying" at the 120V without anyone noticing before someone really touches it and then it is too late.

 I understand .