How do you merge two rabbits together smoothly?

There are good tips here under the Socialization heading and a video of the process as well: http://www.rabbit.org/behavior/index.html Most important thing is that both rabbits are altered (spayed or neutered), and that you make the introduction on neutral territory that neither has claimed. Do short, frequent sessions, supervise closely, and immediately stop any fighting. The more bad experiences they have with each other, the harder it will ultimately be to bond them, so go slow and make sure they are both ok with the speed at which you progress. Wherever you house them together ultimately, make sure it is large enough they can get away from each other when they would like to, and that there's a hiding place somewhere as well.