User blog comment:IamJakuhoRaikoben/Chapter 328: Zodiac, Review./@comment-189.26.99.42-20130419232226/@comment-7418318-20130420073920

Lucy already summoned 5 Zodiacs and 1 silver spirit (or 4 in the anime) without even touching her keys once before and that was before taining, experience and second origin made her a lot stronger, so it isn't that unlikely that she might be able to sustain six of them for a while (assuming the energy drain was split equally between the two mages).

In addition there might also have been a similar effect to unison raid, where the combined power seems to be greater than the sum of it's parts.