Sometimes girls pee on boys if the boy is trying to mate with her and she doesn't want him to. If the guinea pigs are the same gender then one could be annoying the other.
If they are male and female, and you do not want the female to get pregnant, then IMMEDIATELY take the male out of the cage and put him in a different cage for a few hours. If you WANT the female to...
He is expressing dominating behavior towards the other. This can also include "mounting" the other. Don't worry, this is quite common. My guinea pig sometimes does this to his brother who he has...
Yes, though make sure your pet doesn't have any illness that could infect other guinea pigs. Even putting a male with a female or vice versa could be a problem, as the female could come into heat and...