Two boars in a group usually won't work as fights are pretty much inevitable; neutering only takes away the ability to make babies but it doesn't stop their instincts and dominance behaviour. With very rare exceptions (the boars need to be REALLY good friends and both on the laid back side plus you'd better have tons of space to allow for several mini group territories), the rule is one neutered "husboar" per sow group. Guinea pigs are wired for sows to choose the boar they want to associate (and procreate) with and to form a social unit, not for boars to live together in one group with all sows shared between them.
It is not unusual to want having all your piggies living peacefully together, but that works only with larger numbers and lots of space; rarely in small groups with limited cage space. The wounds from a full-on fight between boars can be severe and if very unlucky even fatal; it is also very stressful for all piggies involved, as I can personally testify when groups of mine have accidentally met when grids between pens have come apart.
If you want to see a most amazing, purpose-built set-up that works, have a look at this German facebook page here. Please note that the whole garden area is fully enclosed (including aviary grids against aerial predators and that the piggies also have got some indoor space for cold weather/frosts). The lady is not a breeder, by the way, and all males are neutered.
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