I don't really see why this would be banned in Double Battles. I'll explain my reasoning later on.
For singles, the SkarmBliss combo is a potent, although dying team combo. Blissey has incredible Special Defense while Skarmory has incredible Defense. Working in tandem they can absorb both spectrums with ease and wall out the foe. Unfortunately, this has become less viable as the overall power of Pokemon has increased (see Guts Conkeldurr) as well as the introduction of Psyshock.
As for Doubles, I see no idea why this would be banned. It is not very viable as your opponent can easily target the one that will be more impacted by your attack. I would not consider using it in Doubles.
Blissey alone can be useful in Doubles though. While you still have to keep it away from physical attacks, it has a large support movepool consisting of moves such as Heal Bell, Gravity, and Heal Pulse that can prove helpful.