My guinea pigs, behavior?

ok so i currently haveu two guinea pigs, im pretty sure they are both females, but i have had my one guinea pig for about a year and her name is vanilla, and recently bought a new guinea pig brownie and have had her for about a month or two. but since females are supposed to bond i put them together for a play date for a couple times here and there for them to get used to each other but im not so sure they are getting along. vanilla is assurting dominance in the cage and on the open flor space, giving nips and bits occasionally when face to face with brownie, and one time brownie got niped hard enough for a loud wheek. we checked for injuries but she was fine. i am still very concered for them to be together. i have them in seperate cages for the time but eventually i want them to be together happily. i just want to make sure they dont fight, how can i achieve this goal? please help!! P.S., not entirely sure if vanila is a girl or boy but bought it at a petstore