When you do "Obj on collision with Obj", it picks two instances. And you need to do additional picking if you need to get the type of each of them - use "System pick Nth instance" or "System For Each".
You can create two families with the same sprites to make things easier. Then you could refer to each instance without having to pick them one by one. For example:
Fruits1 On Collision with Fruits2
Fruits1.animationName="Blueberry" & Fruits2.animationName="Blueberry"
-> Create Cherry
Fruits1.animationName="Blueberry" & Fruits2.animationName="Raspberry"
OR Fruits1.animationName="Raspberry" & Fruits2.animationName="Blueberry"
-> Create Pineaple
If there are a lot of fruit combinations, this approach is not ideal because you'll have to create a lot of events. If this is the case, I would suggest using an array or JSON to store all these recipes. And perhaps a function which will look up a pair of names in the array and return the name of the merged object.