You can't just rip a bonded boar pair apart, stick them with sows and then back together in a tiny breeder box; that doesn't usually work. Once their has been a fight with full-on bites to the face, neck or rump, boars will usuallt not go back together.
Any single guinea pig in a good standard is being carefully assessed as to whether they are fit to go with another of their sex or not. The ones that do, will be often used for dating as companions with bereaved or single adopters' boars or bonded with another single rescue boar.
The ones that do not get on with other boars are often neutered/de-sexed and paired up with sows.
This is generally the practice in good standard rescues like we recommend that take character compatibility into account and offer to introduce the boars under expert supervision at the rescue to see whether they get on or not, so you do not come home with a guinea pig where acceptance has not happened. Rescues that cannot offer this service in the UK (like many RSPCA or Blue Cross branches) have a strict neutering policy for any incoming single or fallen-out boars; they only rehome them to live with a sow or several.
Please be aware that anybody can call themselves a rescue or shelter without licensing or welfare supervision and the results can be accordingly.
Can you please specify what scenario you are contemplating? If you are thinking of rehoming from one of our recommended rescues, then you can generally trust their judgment and will have their support if things don't work out.
PS: My Carwyn for instance is a neglect breeder boar who did fall out with his brother (his disfigured lip gives him a very special charm). He has spent the last three years (half of his life) as a very happy 'husboar' living with sows after he was neutered by the rescue before being rehomed.
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