Summarize the following:
You should keep your guinea pigs separate from one another in a different room for the first two to three weeks. You will need to have the new guinea checked out by a vet during this time to ensure they are healthy. Do not let them have physical contact until this time period is up.  Placing the new guinea pig directly in the cage with a new one can cause undue stress as they must be eased into a new environment, and their presence may cause the current pig to become territorial. In addition to the emotional concerns, many guinea pig illnesses, which are easily spread at pet and department stores, can incubate for long periods. You need to make sure your new guinea shows no signs of sickness before introducing them to your old guinea.  Put the guinea pigs in separate cages next to each other. Position the cages in such a way that the guinea pigs cannot see each other but can smell and hear one another. Once the two to three week quarantine period has passed, it's time to introduce the guinea pigs. However, do not place the new guinea pig in the cage right away as it's best to introduce them on neutral ground so neither guinea pig feels their territory is being threatened.  Find a new territory where neither guinea pig has been before, but that is a secluded and quiet place where both guinea pigs will feel safe. On the floor of a closed-off and small room, such as a bathroom, is a good option.  Put veggies, treats, and hay in the center of the area to distract the guinea pigs from fighting as they get accustomed to one another's presence.   In case the guinea pigs get physically aggressive, you should have old towels on hand. You can use towels to restrain the guinea pigs while avoiding getting scratched yourself.  If it goes well, and the guinea pigs have not fought for over two hours, you will be able to introduce them to the same cage. Ensure that the cage has been thoroughly cleaned, and all the toys re-arranged to make it seem like a new cage. After the guinea pigs have been introduced, there are steps you can take to smoothen the transition before placing them in the same cage.  Make adjustments to the cage. Guinea pigs are territorial, so make sure your cage has enough space. As a general rule, a cage between 7.5 and 10.5 square feet is recommended for two guinea pigs. More room is preferred, and if you're housing three or more guinea pigs aim for at least 13 square feet. Rearrange and wash the cage so it will smell new and feel more like neutral territory for both guinea pigs.  Gently rub hay from the old cage on the new guinea pig so he smells more like the rest of the herd.

Summary:
Quarantine the new guinea pig. Introduce them on a neutral ground. Take precautions before putting the guinea pigs in a cage together.