Is one of my guinea pigs a female?

2 days ago i bought what i think are 2 male guinea pigs, the pet shop said and checked they were both male. ive noticed that every now and then one of them starts making a deep purring sound and wriggles about slowly around the other one. i saw that if a male guinea pig does that then it is trying to attract a female in some way. my friends have said that the pet shops can make mistakes and think they are the gender that they are not. is one a female? thanks!