The back of the cover only lists some generic names, and for that bit you are looking for, I think they name them Madoka and Risa, should be one of those two (but that doesn't really help at all I guess)
岡山涼花 from TYOD-024
those are all Cover images from ran maru releases, so if you want to find all the others I suggest looking on http://www.ran-maru.com/search/ (on the lower end you can look by month/year etc.)