Set up a large, penned-in area outside of the cage. Don't just throw the newcomer into the cage--you need to let them get to know each other in a neutral area. Guinea pigs are very territorial and tend to scent-mark their areas, so they may be more hostile to a newcomer in a place they consider "theirs."
Place some vegetables (lettuce, bell peppers, carrots) in the neutral area as a distraction.
Put both guinea pigs in the neutral area. Let them find each other.
Be prepared for some normal establishing of dominance. Rumblestrutting (waving the rear end from side to side and emitting a low, loud purr) is completely normal, as is mounting and the occasional scuffle. They need to work out the issue of dominance, so don't interrupt this. Even a little fighting is okay, as long as they do not draw blood.
Clean the cage completely. Scrub the floor of the cage and any accessories with vinegar to neutralize any existing odors. Add a layer of fresh bedding to the cage.
Put them in the cage together. There may still be a little scuffling as they re-establish dominance in their new surroundings, but it shouldn't take long.
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