In addition, SplatNet 2 and SplatNet 3 do not sell gear with Ability Doubler and Splatoon 3 does not allow installing Ability Doubler with ability chunks (preventing Ability Doubler from being reinstalled for the rest of the Splatfest if overwritten).
Mechanics
This ability doubles all ability points (AP) of the secondary ability slots of the Splatfest Tee, making them provide 6AP each instead of the usual 3AP.
Hence, Splatfest Tees with Ability Doubler provide 18AP (2×3×(3AP)) maximum each instead of the usual 19AP (1×(10AP) + 3×(3AP)) maximum.
Tactics
Ability Doubler can be used to achieve certain combinations of ability points that are not possible without this ability, such as 6AP (2× subs) of three different sub abilities. Abilities whose effects are significant with a low amount of AP (such as Swim Speed Up, Special Saver, and Ink Resistance Up) should be considered for this use case.