In games, it's all about random low queue performance and access time. Nearly every application loads the most frequently used data into RAM, and later, it uses it while additionally loading a lot of random, small data. This is why when you start the game, enter a new location, or start a match, it takes a couple of seconds to begin, and later, it runs smoothly as the required data is in much faster RAM. I don't know how to explain it in an easy way, so you could imagine that.
Either way, it's recommended to install the OS on an SSD, but it doesn't have to be the fastest one, as it doesn't really matter if the OS boots for 2 or 4 seconds. On the second SSD, it's good to install games or demanding applications to separate them from the task queue of the OS storage.