You'll need a really big enclosure (like a full room) that encompasses basically several territories and triggers a herd situation rather a than a group situation and even then it doesn't necessarily work out. We have one member with a large enclosure and several neutered boars in with sows, which resembles more of a herd setting than the territorial small group setting that our usual pet cage situation evokes.
In a normal cage situation, you have to go with one 'husboar' per cage per bonded sow group for a long term stable situation. The various mixed gender groups can live in adjoining cages. Positive outcomes of two boars with a sow group are rare; they need to be very closely bonded and very laid-back. What you are aiming for falls basically into the gap between group and herd in socially interactive terms.
If you want to experiment, you very much need to have the necessary cage back-up/extra grids if you have to split very, very quickly when a smouldering situation suddenly blows up. Interpersonal conflicts are very stressful for the group and for the whole herd from my own experiences.
Boars growing up in a room with sows produce a calming compound and do generally not overreact to sow
pheromones the way when you introduce sows into a previously boars-only space but all the other considerations still apply - personality outlook re. dominance, character compatibility, teenage hormone spikes etc.
Then there is the phenomenon of 'mega-seasons', which you see only in a room with lots of sows where the pheromones from a strong sow season can trigger all other sows close enough in their own oestrus/estrus cycle to come into a strong season as well in a kind of domino effect over the course of 3-4 days with a really palpable cloud of female pheromones that can set off even well bonded boar pairs in the same room. Over time, the sows will group into one larger group having their season at a similar time and then the smaller countergroup having theirs at the counterpoint of the larger group's cycle.
Mega-seasons were the times I had to do a short term separation for my teenage bachelor boys and one pair which was just about hanging in the balance over a personality match ended up parting ways in an 'amicable divorce' (a split without fighting after the dynamics headed firmly into the wrong direction and fronts were firming up). These are my personal experiences in a room with lots of sow or mixed gender pairs and groups and the very odd boar pair.
I have done my own fair share of experimenting over the years but the two boars group has never worked out for me. Even with the most laid back boys and sows, the stress level between all piggies involved was too high; telling me that they were not happy with the situation even without any outright aggression.
