Double Battles work differently than Single Battles.
Think of it this way, where the battlefield looks like this:
Assuming Shaymin-S knows a flying move, both Azumarill and Shaymin can shift their focus onto Blaziken, KOing it and making it not become a threat.
Likewise, Blaziken and Vivillion's Bug moves can be targeted at Shaymin-S, KOing it and making it not become a threat.
In Single Battles, Shaymin-S might not be able to KO Blaziken on his own, leading to a Speed Boost and making Blaziken become a sweeping threat.
All and all, double battles can put more focus on the "uber" Pokemon, therefore having an easier time with them.