Now this is probably why Niantic took so long to add Ditto into the game, its confusions like what does Ditto transform into if it is defending a gym and there are two attackers? What happens when Ditto fights Ditto? What happens when a trainer enters a battle with a Ditto while another battle with a different Pokémon is taking place?,
Well here I am going to try and answer for you, with what other trainers have experinced while using or confronting Ditto in gyms.
First of all as I explained in another article earlier this week, when Ditto fights Ditto nothing happens and they both just simply don’t transform.
But as for the other question many Pokémon Go players have asked about, what happens when Ditto is defending and there are two trainers trying to capture the gym? Well then obviously Ditto transforms.
But which one does Ditto transform into? Well through observations sourced from trainers on a number of forums, it picks the weaker Pokémon to transform into.
There was a case where a two trainers, one with a Vaporeon and the other with a Rhydon attempted to take down a gym with a defending Ditto. The Ditto decided to chose the Rydon to transform into despite, the Vaporeon being stronger.