Well done for arranging for her to have a friend.
As one of the piggies is a baby, then bonding needs to take place straight away. Babies under four months of age must never be alone because they are desperate for companionship and guidance from another piggy.
Bonding is a one time event - you find a neutral territory, somewhere neither of them normally go, and put them into that space for several hours. They go through various stages of forming the rough outline of their relationship in the bonding pen. If things go well during those few hours, then you thoroughly clean down the cage they are to live in so that it is also neutral and then you can move them together into the cage to live together from then on. It takes around two weeks of permanently living together for them to sort out the fine details of their relationship.
As any bonding comes down to character compatibility, then an on spec bonding (whereby you simply buy another piggy and plan to introduce them without knowing anything about the new piggy’s character) does come with a risk of failure - they two of them may simply not like each other. In this case, the bonding will fail and they will need to live in separate side by side cages permanently as neighbours only.
However, the fact your older girl has had no social interaction could make things trickier - it might not though, all you can really do is follow the steps as listed in the first bonding guide below and see what happens.
Ive also attached another guide which details fear aggression just in case your older piggy has socialisation issues. As I’ve said, normally, bonding is a one time success or failure event, but with fear aggressive piggies you can do a slower bonding - the guide explains.
Good luck, ask questions if you need to and keep us posted.
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
Sows: Behaviour and female health problems (including ovarian cysts)
Moody Guinea Pigs: Depression, Bullying, Aggression, Stress, Fear and Antisocial Behaviour
