I have seen cows {females} get onto each other as well as dogs {females} sometimes it can be a dominance thing or just a bit of overactive hormones.
Or you could have a boy & a girl...

Sexing Hamsters - How to Tell the Sex of a Hamster
Sexing hamsters can be difficult, especially if they are young or you don't necessarily have both males and females to compare.

The basic differences are similar for all species of hamsters, Syrian, dwarf, or Chinese.

Hold your hamster on its back to look at the belly area. You can cup your hand over your hamster and gently turn it over onto its back, or hold the hamster by the scruff. Holding the hamster in a slightly upright ("sitting up") position will help you determine if testicles are present.

Anogenital Distance
The surest way to tell is by looking at the anogenital distance. This is the distance between the genital openings (penile opening in the male, vaginal and urinary openings in the female) and the ****. This distance is much shorter in females than males (in fact the openings can be difficult to distinguish in females while they are separated by about half an inch in males). This is illustrated in the diagram below.


Other Differences

* The back end of the male tends to be elongated and rounder in appearance in comparison to the female (especially noticeable in Chinese hamsters).
* Males often have a noticeable scrotum with testicles after about 4 weeks of age, although the testicles can be retracted.
* Only females have nipples


Sexing Hamsters - Diagram - Female or Male Hamster
Sexing Hamsters - Diagram of the differences used to tell if you have a male or female hamster