This way they both smell the same. This will help them get used to each other quicker. Introducing females to females Female rats usually get along with others very easily. A very few females are extremely territorial, but most will readily accept new girls on their turf. I have had no problems with females getting along and all have readily accepted new friends without a squabble.
Male rats are very reluctant to accept newcomers, with one exception.
If you introduce a 5-8 week old rat to an older male, the older one will adopt the younger as his son and will be very gentle with him. Later, after puberty hits, it is much harder for male rats to accept others. Even males that do get along will occasionally become so territorial at puberty that they will beat up their former friends. Males usually always fight to some extent and do a lot of boxing, power grooming and flipping of subordinate rats. Male rats seem to enjoy playing rough, but if one is getting picked on too much or is getting hurt or scared, it's better to separate them.