No, you do not get EVs for defeating your own Pokémon in a double battle. I knocked out my Slugma in a battle against two Spinda, using PkHEX to view my Breloom's EV spread before and after the battle. This battle only awarded Breloom with 2 Sp. Atk EVs from the Spinda, and not an additional Sp. Atk EV from Slugma.

Note that Breloom's total Sp. Atk EVs only go from 11 to 13.


Also, it's less resource-heavy and/or time-consuming to simply EV train against wild Pokémon instead of your own.