I’m sorry about the loss of your bunny. Three month old bunnies are not too young and they don’t need 24 care. They would need pretty much the same care as an adult rabbit. Their diet would be a little different.

You will of course have to keep the two separated until they have had their spay and neuter surgeries, then go through the bonding process.

All domestic rabbits are European rabbits.  In the wild, momma delivers the babies in a separate birthing nest that is not the nest where momma sleeps.  The mother rabbit visits the new born rabbits once/twice a day for nursing.  Rabbits are from the get-go accustomed to long periods of being by themselves. In my 12 years of living with free-to-roam house rabbits, I don’t think there’s been any concern about leaving them being unhappy about being alone. 

I think the major caveat is that rabbits need to be monitored at least once every 24 hours.  If they develop a gastric problem…don’t eat….they can die quickly.  Rabbits are basically very robust animals, but their digestive system is actually very fragile.

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With the advent of home surveillance cameras, people have discovered that surprisingly dogs DO NOT like to be left at home alone. Single dogs, alone in a house, can become very agitated.

There are cases when buns when buns won’t bond, but that is more common with older bunnies who have been living solo. Bunnies who have been friends before puberty will as a rule bond very well after de-sexing and healing, as long as you have separated them before any heavy fighting has begun. With 2 12-week-olds, you’d of course have to separate right away in order to prevent pregnancy. But if you keep their cages in the same room, preferably so they can see each other (but not too close, they can mate through cage bars), they will become familiar with each other and the likelihood of a smooth bonding precess is big. You can swop their litter-boxes between them, swop blankies and toys, that will further familiarize them with each other.

12 weeks is not too young. A 12-week old rabbit is self-suffcient and doesn’t need it’s mother. As flemmishwhite says, baby buns are left pretty much alone by their mum except for feeding when they are little, so they’re not like cats or dogs or rats and not at all like human babies. During the day, most buns sleep or lounge, they are crepuscular animals, meaning they are at their most active during twilight. If you have two buns who can see, hear and smell each other even if they can’t interact directly, they will take comfort in the presence of each other.