Seems legit; the Calamity effect and the loss of a ring slot definitely balance the fact there are two health bars fighting one IMO.
But as the other posters have said there are still two weapons coming at you at the same time, which means that overall the fight is still going to be unbalanced from a 'fairness' POV.
OP mentioned he was considering adding other restrictions/handicaps, so I'm sure he'd appreciate input from people thinking that this match-up is still unfair.
As I have said, I think the double damage and -1 ring slot more than makes up for the discrepancies in the HP bars.
The other areas I think you would have to adjust would be armor/weapon restrictions (the combined poise ratings, offensively and defensively would be a big advantage in a 2v1 match). For instance you could limit armor on the team of 2 to light/medium sets, and disallow Iron Flesh Pyromancy. With weapons I would personally limit it to the lower 2 tiers of poise damage, the weapon classes being Curved Swords,Katanas, Straight Swords, Whips, Spears, Polearms, Rapiers, Daggers and Fist Weapons. Also disallowing spell buffs (and possibly resins) and Power Within.
These limitations would need to be tested for balancing, but I think that would be a good general place to start.
Anyway, theres my $0.02 OP. Good idea too man, thanks for bringing it up.