I personally wouldnt advise it. Boars can get on in pairs just fine but it is a tenuous relationship. The fact that they have lots of room is probably one of the reasons they do get on. You cant add a sow as she will get pregnant and even if you got the boars neutered, they will fight over her and you cant really add another boar as this will upset the balance and they are very likely to fall out, resulting in separation, possibly for all three!
The only thing I can suggest is, if you have room, getting another hutch and getting another 2 piggies for that.