Please can you explain what is actually happening between your two boys.
You use the words aggressive and fighting.
If the boys have had an actual fight then their bond is unfortunately a failure. In this case, they will need to live in separate cages permanently.
If it was not an actual fight and it was just dominance, then you can try to reintroduce them but do so on neutral territory and see if they want to go back together. If they do well in a bonding pen, then as before move them back to the cage.
If they do go back together, please do not separate for mild dominance behaviours in future. It is unnecessary and can in some situations disrupt bonding.
If they choose they no longer want to be together, then obviously they would need to remain apart and live in separate cages permanently.
I know your two piggies are male but what sex is your niece’s guinea pig?
If it is a female then that would be why the boys have started fighting. You should not bring a female guinea pig into the room where a bonded pair of boys live. Sow pheromones can cause bonded boars to fall out.
If your niece’s piggy is a male then having another male in the room should not cause your two boys issues in itself. But in a new bond, you’ve piggies it may have caused some excitement for them.
However again, if your boys have had an actual fight then the outcome is the same regardless of whether your niece’s piggy is male or female - your boys are likely to have had compatibility problems which suddenly came to light (which can happen during the two week bonding period when the fine details of a bond are ironed out) and their bond unfortunately is a failure.
