Usually it's better to have 2 babies of opposite sexes and get the male neutered to prevent unwanted litters(if they breed, you'll have to seperate them, and then they won't keep each other company). Are they actually fighting? It not be that they're not getting along, it may just be that the new pig is noisy! Guinea pigs are amazing at how many different sounds they can make!!! I've had some very quiet guinea pigs, and others that just won't shut up!!!
I have had vet tech training and worked in vet hospitals, but I am not a vet-please go to a vet for serious problems!
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