Double Feature by Rosamond du Jardin (1953)
Pretty Pam Howard is still taken aback by her quiet twin Penny's new attitude of independence. Until recently, Pam led the way, and Penny followed. Now Penny wants them both to go to the college that her friend Mike plans to attend, but Pam is resisting--partly just for the sake of resisting. Old field marshals don't give up easily!
Oddly enough, the ensuing fireworks strengthen the twin's relationship, and the college of Penny's choice proves an exciting place for Pam. also. As a matter of fact, for a while, it's almost too exciting! (from the back cover)