I Devised Some Double Battle Rules for the TCG

Espurration

In a TCG Double Battle, each player has two Active Pokémon and four Benched Pokémon. They are your Left Active and Right Active Pokémon, and like in a TCG Triple Battle one player's Left Active Pokémon corresponds to the other player's Right Active Pokémon and vice versa.

The Rotation Mulligan rule carries over as a Double Mulligan, but the minimum number of Basic Pokémon required to be in your starting hand drops from 3 to 2.

There is no prohibition on Bench expansion in a TCG Double Battle.

Either of your Active Pokémon may attack either of your opponent's Active Pokémon.