User blog comment:Water Dragon Slayer/DREAM BATTLE CHRONICLES: FIRST MATCH!/@comment-11966702-20130717123030/@comment-7418318-20130718082629

As far as we know Yukino doesn't have her 2nd container unlocked.

And yes it is possible to gain more magic power through training (Lucy even did this with Capricorn before being "invited" into the Celestial Spirit Realm), so Lucy would most likely have gained the ability to summon two spirits without collapsing even without Ultears help sooner or later.

But such training would in all likelyhood take more than just the two days they were able to train before the GMG.

Yukino said that she worked hard untill she was "finally able to get in" Sabertooth.