The two designers -- one Uptown and one Downtown, and longtime friends -- came together to create a capsule collection of swimwear called Shoshanna Made With Love Charlotte Ronson.
"Charlotte is one of my oldest and closest friends," Gruss tells StyleList.
"We went to school together [at Manhattan's Nightingale-Bamford School], we've been in business almost the same amount of time, and we've always been very supportive of each other. We don't have the same style, but when we shop, we always like the same things."
The collection grew out of their love of the mini prints Ronson used on her defunct line of boy-inspired underwear, Tooshies.
"We wanted to do something that was a crossover from my customers to her customers," Gruss says. "Last year, it worked great, and it was really fun working together." So this year, they're back for more.
"It's a great little collaboration," Ronson tells StyleList. "Shoshanna has the swimsuit thing down pat. We're both kind of drawn to the same sort of feminine prints, but her look is a little sexier and mine's flirtier. We just get to play around."
The collection hits stores this month and includes matching Mommy & Me bikinis and cover-ups in three prints, as well as a limited-edition beach bag.
Gruss started doing Mommy & Me swimsuits when her daughter, Sienna, was born in 2005. However, when she and her nearly five-year-old don the suits, "I'm more into it than she is," Gruss admits.
Read about Jessica Stam and Rachel Roy's collaboration on a capsule collection for Macy's.