First off, for at least 2 weeks the new piggy needs to be quarantined for both of their safety. You don't want them to get sick do you? Then put the cages on opposite sides of the room so they can smell each other. Leave it this way for a week, then put the cages right next to each other for one week. If they do not show aggressive behaviors, then they can be housed together in a 10 square foot C&C cage (http://www.guineapigcages.com/). If they act aggressive when they are together, then leave them next to each other for another week. Also, when they are housed next to each other, make a split C&C cage and just remove the split-cubes. Do not put one guinea pig in the other's cage or there will be fighting.