in Doubles. Would Stomping Tantrum still get the power boost if I miss Rock Slide on only one of my opponent's Pokemon? I couldn't find anything that explains this exact scenario.
No, Stomping Tantrum does not get the boost in power if Rock Slide only misses one target.
From my Showdown testing, Stomping Tantrum deals the same amount of damage, regardless of Rock Slide missing the target of Stomping Tantrum or that Pokémon's ally.
Showdown Replay - Misses Ally Showdown Replay - Misses Target
No
It did connect with the opponent. Simply just not both but it did hit making stomping tantrum normal.
Source 1 Experience and common sense. Source 2 Source 3