Introduce them first with their cages next to each other so they see each other. Then introduce them out of the cage. Have a spray water bottle by just in case they start to fight which can be common with males. The water will shock them so they stop fighting.
I would recommend getting a new cage because if you put your new guinea pig into the other guinea pigs cage, the older one may get terretorial of his space and this could cause problems. If they don't like each other you will know because they will hiss or fight.