Your question is not clear enough to be answered. Please describe what you're trying to achieve in detail.
You have two mains voltage light switches? What type (single pole, twin pole)? If they are already wired up so that both switches control the same lamp(s), they are probably twin pole.
You want the Arduino to replace one of those switches, or both? You want to have the two switches as inputs to the Arduino?
Before you get too far down this route, also try to be clear about how the logic level and mains voltage wiring are physically and electrically organised. Even the faintest remotest most hypothetical possibility of mains voltage being connected to your Arduino hardware is a potentially fatal problem that needs to be addressed. So, if any part of your Arduino is going to be connected to or placed in the same enclosure as any mains voltage hardware or wiring, you have a problem.