It can be done more gradually, starting with first swapping items and putting them in each others area so they get used to the other rabbits scent. If they are acting aggressively towards one another, it might be best to slow down this bonding process and not put them right together just yet. At least until you've read up on more bonding info so you have a better idea of what you're doing and what behaviors to look out for and when it's essential to intervene so a fight doesn't break out. Because once a fight occurs, chances of having a successful bond ever, will decrease significantly. See if you can access some of the info from this link below. It gives a good description of different bonding techniques and the different behaviors to watch out for.
wabbitwiki.com
(and some videos of aggressive behaviors to watch out for)
Bonding rabbits together
(and some videos of aggressive behaviors to watch out for)

BONDING Bunnies : CottonTails Rabbit & Guinea Pig Rescue
INTRODUCTION TO BONDING METHODS Most rabbits are territorial by nature. However, this does not mean that they all have to lead solitary lives. Rabbits that live in compatible pairs or occasionally even small groups will often benefit in many ways, including companionship, mental and physical...
cottontails-rescue.org.uk