Hi, you just described what happened to be 3 years ago, I introduced my 3 year old male and a new young male guinea pig. Females as a rule are generally easier to introduce then males, jdont just put one in the cage though, you shouldn't put them together for the first 2 weeks incase the new one is ill and causes the older one to to become ill. Then dont just put one in the cage of the other, introduce them whilst holding them in a different room, and monitor them closely so you can seperate them if you have to, and try again untill they accept each other, which they usually will
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