There is a few different ways you can tackle this but the concept is you have a variable (either a global, local or an instance variable built into the object).
On collision or overlap, you want to "Set a value" for this variable and add in the randomise part. Eg : Choose (1,2).
Now you make 2 seperate sub-events (events that are directly below and indented) and "Compare variable"
Variable = 1 -> Do this
Variable = 2 -> Do that.
If that doesnt make sense yet have a closer look at the manual (there is also a ton of examples just use the search function).