There are many threads on here about bonding and I'd recommend you read through a few of them as the answer to your question is huge.
Depends on many many things.
Most importantly you need
bunnies to be neutered.
neutral and confined space to do it in
Time to do it and be able to watch constantly
rabbits that suit each other's temperament
If you have a look you'll see lots of experiences that others have had.
If you'd like a leaflet that speaks about the way I do it then please PM me.
Helen