So this is a pair? Is it the larger or the smaller that's nipping at you?
What type are they?
Like Meowzer stated, they might be mating.
What size is this tank and have they always been alone?
There are a few things that can help with aggression, especially with fish that have established territories:
-Rearrange your rock work (it's a PITA, but many times this solves the issue, especially with paired clowns, they're too busy trying to find a new home to worry about new fish)
-Add fish after a feeding and while lights are out
-Add fish and then feed (this works well for me, my fish are pigs and would rather eat than worry about a newcomer)
My Ocellaris clowns are very aggressive. The female is relentless right before she lays eggs. I put them in the refugium (30G) because I was tired of them biting my fingers every day. They're in an anemone, so when I need to throw another fish in the refugium, or I need to move things around and don't feel like being bit, I feed them small chunks of food that they keep bringing to the anemone and that keeps them somewhat occupied. It only buys me a few minutes, but in a refugium, that's usually all I need.
