They can't breed
Pokémon in the Ditto group can breed with any Pokémon from any Egg Group, excluding the Undiscovered Egg Group. However, Ditto cannot breed with members of its own group, meaning that Ditto cannot be hatched from an Egg. The only exception is in Generation II.
Ditto is a genderless Pokémon, so its the same you breed a Mewtwo with a Mewtwo and you will get nothing as you may know because Mewtwo is a genderless Pokémon.
When Ditto is breeding with a male or a female Pokémon, it will ''get'' the other opposite sex of the partner making a egg.