User blog comment:Water Dragon Slayer/Chapter 348 - The Demon's Back/@comment-33019320-20130823091518/@comment-4829046-20130823101615

It would be weird if they could change ownership temporarily, like when doing missions the two of them split the keys according to the nature of the mission or something or maybe even have separate time-slots like Lucy can summon Capricorn on Wednesday at noon and Yukino can summon him on Saturday so when going on a mission on a Tuesday Lucy will have him. But then they would be unprepared for the unexpected in which case they would get whooped so I suppose this way is best >_>

Yukino will reappear in a future arc anyway.