I think it makes sense and is less than a Deus Ex Machina than I originally thought. Lucy has been building up her magical limit (I guess that's what we call it) with the constant use of summoning two spirits at once. It's already a feat that she can do that so if she keeps doing so then she'll be able to summon more. But what's squashing my main quibble about Mashima pulling this out his ass was the fact that she lost Aquarius. In order to gain a more powerful spirit ONCE. It wasn't a 'oh how in the hell she get the energy to do that', it was a fair trade off. In order to summon the CK she had to break a key, and I think she had to dismiss Loki and Aries as well. So it's a fair trade of energy. Three spirits + contract = One CK.
But that didn't make my feels hurt any less.
