A Capella does not gain the boost (2x) when there is another Pokémon on the team with it. I can get a log of this where a Specs Porygon Z did more damage with Tri-Attack than with A Capella if necessary.
The other Pokémon with A Capella was not fainted, and in fact had not even entered battle yet, when A Capella was used.
The other Pokémon with A Capella was not fainted, and in fact had not even entered battle yet, when A Capella was used.