In the equipped items menu there is a 2nd slot for a pet. Is there a stage to the game when you can equip 2 pets at same time?
No, that slot for the Cup u get from PVP League, it’s only have effect on Arena.
It’s not for a pet, but for a trophy.
In order to get a trophy: play arena ~> win matches ~> get chests ~> good luck
The Good Luck part.