Guinea Pigs are pack animals so they need a companion, but they are also dominant and there is a requirement for one to be pack leader.
If you have an older Guinea Pig, you may like to pair it with a younger one, however, you really need to know your current Guinea Pig's behaviour so that they are matched well. That means one needs to be dominant and the other subordinate.
Size can play a role in who is boss out of the two, usually the biggest one is the dominant.
The size of the Cage is very important to allowing enough space for the two Guinea Pigs to meet and bond. The cage really needs to have a 'time out' area so that if things get tense, they can have some down time to regroup.
When introducing two Guinea Pigs, it is best done on Neutral Territory. That means thoroughly cleaning the cage, the bedding and any other accessories so that there are no conflicting smells. They naturally mark their territory, so it's best to let both have an equal footing.
According to Guinea Pig Australia, it is best to have two, rather then one. The reason is that with a companion of the same species, they can express themselves fully, they are pack animals and they can show you all the fabulous antics that only a pair of Guinea Pigs can get up to.
They suggest that you adopt two of the same sex. If you do get a male and female pair it is very strongly recommended that the male be desexed so that no babies come along that could end up in no-home situation.
There are many Guinea Pigs that need a new home, so "Opt to Adopt."
What about food? Nutrition is important for the health and well-being of your Guinea Pigs. They require a well-balanced diet and to be fed twice a day. Their diet consists of: