There are literally thousands of examples in this forum.
You have 2 "paths" to go (set 1 and set 2). I assume you have setup both sets already in your survey.
The only thing you need to do, like Joffm described is to put a equation question at the beginning of your survey with a random number that is either 1 or 2. So it is the random number process that determines into which set you guide your participants. For all those where the random number comes out as 1 it goes into the set 1 and for all where the random number appears as 2 you guide them into set 2.
Here for example is an description of how to create a random number in an equation question (the rest of the problem is a little different):
survey-consulting.com/how-to-randomly-sh...a-limesurvey-survey/
Here is a description of what you are looking for, but you should not use their approach of how to create the random number, as it is flawed (the way they use it, the random number could be triggered more than once):
www.ryananddebi.com/2015/04/09/limesurve...xperimental-designs/
But if you search in the forum for random number, you'll find a lot of examples as well.