Summarize this article:

The first few weeks for guinea pig cohabitation can be rough, and you should expect some tension between your pets. Know what the signs of aggression are and what body language is a signal for you to intervene.  Mounting or jumping over one another can lead to aggression if one guinea pig resists or retaliates. Keep an eye on this behavior, but do not intervene unless it leads to fighting.  Squealing, chasing, and teeth chattering are common in the first few weeks. Guinea pigs may also gently nip one another if annoyed, and such behaviors are necessary to establish boundaries between the guinea pigs. Only intervene if the guinea pigs are biting hard enough to inflict wounds.  If one guinea pig is constantly chattering his teeth, this is a sign of aggression that could grow serious. Watch carefully, but do not separate unless blood is drawn.  Raised hair, especially around the neck, and feet stamping are signs guinea pigs are preparing to fight. This can occur but watch carefully. Do not separate unless blood is drawn. Again, do not separate unless blood is drawn. Two males or two females humping each other is not an intended sexual interaction, it's their method of establishing dominance. Mounting does not necessarily mean your guinea pig is a boar, or mating. Not all body language is a bad sign. There are certain behaviors that are normal, especially early on, and you should be able to recognize these gestures so you do not intervene unnecessarily.  Bottom sniffing and nudging is a guinea pig's way of greeting a new companion. This is normal and non-threatening behavior. Your guinea pigs may also want to establish their territory by dragging their bottoms across the ground or raising their heads up on occasion to expression dominance, behaviors that are normal during the first few weeks.  Guinea pigs sometimes engage in a behavior called rumble strutting. This is when they walk  with their hips swaying and their fur puffed out while making a rumbling sound. This is an expression of dominance and unless it's followed up by aggressive gestures it's usually a normal part of establishing the pecking order.  Don't be afraid of squealing sounds. Although guinea pigs can make shrill noises that may sound like pain, it is just their way of saying that they submit to the more dominant pig. If fights start to get bloody, you need to intervene. Know how to do so safely to diminish harm for yourself and your pets.  Act quickly. Guinea pigs have sharp teeth and are capable of inflicting serious harm on one another. If you notice aggressive fighting, separate them immediately. Guinea pigs can cause permanent injury and disabilities to one another if left unchecked.  Do not intervene with your bare hands. An irate guinea pig can cause injuries to you that are serious enough to warrant medical attention. Cover the animal with an old towel or rag or use heavy gloves while separating them.  Separate the guinea pigs after the fight. Keep them in different cages, but in the same room so that they can see, smell, hear, and talk to each other.  Continue handling them with gloves or a towel for a few hours after the fight, as they can remain skittish and aggressive for a while after a confrontation.  Reintroduce them slowly, once again introducing them on a neutral ground with treats and food as distractions. Depending on the severity of the fight, wait a few hours to a few days. Keep protective gear on hand in case the fight resumes. Some guinea pigs may simply not get along, even after you follow the proper protocol introducing them. You need to be prepared for this. It's a risk when introducing a new guinea pig to your pack.  Do not feel as if this is your fault. Nature simply works this way, and guinea pigs have different personalities. Some may be more independent and aggressive and therefore harder to introduce to a companion. Even when you do all the right things, guinea pigs are sometimes simply not compatible with one another.  If the first introduction leads to an excess of fighting, you can restart the introduction process, beginning again with the quarantine period. This gives the guinea pigs time to cool off and forget any previous hostilities. If guinea pigs are simply not getting along, you can always keep them in separate cages where they can smell, see, and hear one another but cannot physically interact. They get the benefits of some social interaction but without the stress of a hostile environment.

Summary:
Recognize aggressive body language. Understand that humping each other is perfectly normal. Recognize normal interaction. Break up fights when necessary. Don't be discouraged if they don't get along.