Three's a Crowd by Marie McSwigan (1953)

"Say," Janet turned to her twin. "Let's get those books and get out of here." She would delay no longer.

"She can't be late getting home," Joby thought. "She doesn't want to miss Zip's call."

Zip again. The name was short and sharp like a clasp knife. Just thinking about it drove it home in her heart. (from the back cover)