What? Those are the rules I've been talking about. Either 1) You have Concentration on one spell maintained by multiple casters, in which case the cost of doing that is it prevents those casters from using other Concentration spells or 2) You have your primary caster break Concentration to cast a second Concentration spell, while a secondary caster maintains the first one. In which case, all you're doing is shifting Concentration to another caster who can't use a Concentration spell themselves (and who might be better able to maintain Concentration if they take a hit), but that's hardly game-breaking. It's two Concentration spells going at the same time, but it's also two casters maintaining Concentration, so no different than if each caster had cast their own spell.