There is a wonderful website called House Rabbit Information and Care - BinkyBunny.com and there an a entire section to rabbit bonding with lots of helpful information. Some people are extremely lucky and their rabbits take to eacho ther right away, and some have a lot of trouble. I recently adopted a second rabbit and in a few months my bonding journey will begin as well.
Before you adopted the second one, was the first one in the same cage/room as they have now? That may be a cause for your problem; when the second one arrived there was no neutral territory. Try putting them in the bathroom, or a pen in the yard [be sure to supervise], or a room of the house that no one has claimed yet.