Hi. I have 2 gensets (diesel generator set), I named them Genset 1 and Genset 2. I want to operate Genset 1 for the first 12 hours, and for the next 12 hours, I switch it to Genset 2. I should add a Change Over Switch (COS) to switch the operation of both gensets, how do I determine the best change-over switch?
The switch you're looking for is often called a "transfer switch."

It's doable, but you need a transfer switch that "breaks before it makes"--meaning that Genset 1's output will be disconnected from the load(s) before Genset 2's output is connected to the loads. THAT means there will be a brief period when there is no power being supplied to the load(s).

All this presumes your gensets are AC, and most likely the generators are three-phase synchronous generators--which means you will need a three-phase break-before-make transfer switch. (If the generators are single phase generators, you will still need a break-before-make transfer switch.)

Best of luck. Use your preferred World Wide search engine to find transfer switches in your region of the world.