I always hear people tell me that balance has no place in rules interpretations, but pish-tosh to them! From a balance perspective, consider the REASON for the rule that you cannot cast more than one quickened spell per round. The reason is because the designers wanted to effectively limit casters to no more than two spells per round (ignoring 3.0 haste and wizards shapechanged into chokers who only get a bonus move action IMC because of that very reason). You get one spell as a free action and one as a standard action. If my 15th level wizard wants to cast quickened Evard's black tentacles and quickened stinking cloud in the same round, what is the true harm if I am limited to two spells. Certainly you wouldn't argue that I could not cast quickened Evard's black tentacles and stinking cloud in the same round so why is it such a big deal to allow me to cast quickened stinking cloud instead when the mechanical effect is exactly the same, and I have to use up more resources the latter way to boot!
Yes, using a literal interpretation of the rule that you cannot cast more than one quickened spell per round limits you to one quickened spell per round period. But if you fail to take intent into account, then you wind up with a seeming contradiction in purpose.