In the picture you can see my 2 Male guinea pigs, in the picture they'd been together for roughly half an hour, and are very happy together. Update(2nd November 2010) : They have been together for 10 months now and are still very happy together!

There is a myth that 2 Males (Boars) will definitely fight if you put 2 that have never met together, this is not true, and it depends on the Guinea Pig. Males tend to be a little kinder to a new comer, but the older one might want to be more dominant.

People also say that 2 females (sows) won’t fight but will be extremely territorial and won’t get on. This is also a Myth, again it depends on the guinea pigs, and most of the time they aren't territorial. A lot of guinea pigs aren't even interested in Dominancy and you will find this happens in a few circumstances. My boars weren't interested at all, until my younger one reached about 3 months old and this is when he began playing for dominancy, and along came a few nose offs, and they started swapping at night in the bed, and took it in turns every night to sleep in their bed, and the younger one tried to mount the older one a few times.

This time takes about a week, then their hierarchy (Which one is more dominant) should be sorted out, and one is more dominant and is higher in the hierarchy than the other, the older one is usually more dominant.

It is relatively the same with males and females.
For about 2 weeks, the New Guinea pig needs to be quarantined. This is when he or she is left alone in a cage by itself.
You will need a neutral place, the best place to do an introduction is on the Sofa, sitting at one end and a towel in the middle, it’s also good to have another person sitting or standing or kneeling on the floor at the other end. Then set the guinea pigs at the sides of the towel, opposite to each other and leave them to make their own way towards each other.

There will be some bum sniffing, nose offs, perhaps a bit of mounting, sometimes there will be a chattering of teeth, and in boars there may be a bit bubbly sounds and pulling of bums.

 - Rumble strutting (The bubbly / purring sounds and shuffling side to side).
 - Bum sniffing
 - Raised hair on neck and part of back.
 - Bum nudging
 - Teeth chattering (a grinding of the incisors being rubbed together)
 - Nose offs. (Raise of heads, they see who can raise the nose highest)
 - Pulling their bum.
 - Chasing each other.

 - A wide "yawn". (Looks like a very wide yawn, it isn't, and they’re showing their teeth).
 - Sustained teeth chattering.
 - A strong Puffing sound.
 - Nipping

 - Bites, not nips, bites with intent to hurt the other guinea pig and draw blood.
 - Raised hair, teeth chattering, perhaps rumble strutting and hunched up with back raised. (Strong sign of possible attack, sometimes they just sit there and do this while staring out the other guinea pig, a warning sign and telling the other guinea pig to back off and go away.
 - Both guinea pigs are haunches up with backs raised with heads right up near each other. Guinea pigs should be split if you witness this.
 - Battle begins. They in locked together in a ball biting each other, and squeals coming from each other.

If they're fighting, throw a towel over them or wear some thick gloves and separate immediately, when in full battle they will bite everything and anything, and if they bite you it can really hurt.

If they do get on, and they've got on perfect together for the first few days of being put together daily, you can clean the cage with vinegar and get rid of all the scent and clean everything, put them in and see what happens.

It’s quite rare for 2 Guinea Pigs to end up in full battle, but I’ve experienced a few of the escalating dominancy behaviour with 2 females. My boars were fine and got on straight away as soon as they met.

The best thing to do is get a young on and put it with an adult, never introduce an adult to an adult. As they won’t ever figure out dominancy and will most likely progress into the escalating and possibly further dominancy signs.

If you have a female, get a female or a neutered male.
If you have a male, get another male, or get it neutered and get a female.
If you have 2 females, get a neutered male or another female.
If you have 2 males, get a neutered female or another male.