When using 50/50 on a marked interval, it will split into 2 equal intervals. If the marked interval is uneven, let's say 21 seconds, the first interval will be 11 seconds and the second interval will be 10 seconds after using 50/50.
Here is a video that will show you how to use 50/50
To use 50/50, you will first need to mark an interval. You will then see the "toolbox" for that particular interval and the 50/50 option will be on the left side of the toolbox.
When you click the 50/50 button the interval will be split into 2 intervals like this: