Magic and Panty and I

From MangaHelpers:

Shiina is a high school girl who has perverted yuri dreams featuring herself and is also a magic user, a secret only her beloved best friend Setsuna knows. Her magic, the “Western Field,” is a power that's supposed to be used to eliminate foes who disrupt society. But one day, she is unable to use her Western Field, only to find out that if Setsuna were to wear Shiina’s panties in her place, Setsuna can wield the powers of the Western Field. With this new development, how will Shiina face her future enemies, which could endanger her beloved Setsuna? [Midori-Mori]