Welcome to the forum and I'm sorry for your loss
Companionship is vital to babies under four months old, so bonding needs to be done immediately on the day you bring the new piggy home. (Settling in/side by side time prior to bonding is only suitable for older piggies).
If bonding on the day of arrival isn’t possible (eg if there isn’t sufficient day time left on the day you bring the new one home) then they can be apart for a night but then you must bond the next day.
If you do need a night apart then keep the piggies in completely side by side cages during that time so they can see, hear and smell each other.
First, double check the sex of the new piggy to be certain they are both boys. (If they are pet shop or breeder piggies then it is always advisable to check yourself as unfortunately we see sexing mistakes too often).
You then put both piggies in a neutral territory bonding pen (or somewhere your piggy does not see as his space). Carrying out a bonding in the cage belonging to one piggy will be seen as a territory invasion and can cause fights.
There should be no hides at all in the bonding pen, just hay and water.
Leave them in the neutral bonding pen for at least several hours but even all day is absolutely fine (moving back to the cage too soon isn’t advisable).
If all goes well after those hours, then you clean out the cage they are to live in together and move both boys to the cage together (ensuring their cage is big enough - 180x60cm). Do not separate them unless there is a fight.
Once in the cage you can then add hides which have two exits (don’t use single exit hides with boys at all - single exit enables cornering and can result in fights). Boars need multiple of every item in the cage.
The full process of forming a hierarchy will then continue and can take up to two weeks to finalise.
If things don’t go well in the neutral territory pen, they dont like each other or are unable to form a hierarchy (either immediately or down the line - a successful bonding comes down entirely to character compatibility) then they will need to live in separate but side by side cages permanently to provide interaction between the bars only.
Unfortunately this can happen with any on spec bonding (you simply don’t know if they are going to be compatible) and while more boar pairs make it together happily than not, it is always wise to have a back up plan just in case of issues.
I have added our bonding guide below
Bonding and Interaction: Illustrated Bonding Dynamics and Behaviours
And this is a guide specific to boars, their behaviours and needs
A Comprehensive Guide to Guinea Pig Boars
Do ask us any more questions if needed
Fingers crossed it goes well
