Do your research. If you only have a minimal amount of experience in stage combat, watch as many filmed or staged knife fights as you can. This will give you ideas of the kinds of moves that can be involved; if you look closely, you may even see the tricks that the actors are using. Part of what you'll notice is that in knife fights, the knife needs to be as close to the opponent as possible while your body is as removed from the action as possible.
Script/choreograph the fight. Both actors need to know the order of exactly what is going to happen and when. It can be planned with an outline of the beginning, middle, and end. There must be a climax, so think about what happens as the fight builds up to the climax and what happens immediately after.
Prepare your rehearsal space. Keep things as safe as possible by rehearsing on a soft surface, such as gym mats or perhaps grass. Have a first aid kit close by and have your props ready. Props are not to be real weapons; never use a real knife in any form of stage combat.
Determine the giver and receiver of each action. In staged combat, the "giver" of an action should never deliver any force to the receiver; rather, it is the receiver who should be in control of the action. For example, if Fighter A is supposed to grab Fighter B by the hair before jabbing the knife in his side, Fighter B should react by pretending to defend himself. Fighter B might look as if he is trying to release Fighter A's hand from his hair when in reality, he is holding B's fist to his own head while A only puts on the appearance of grabbing hair.
Go slowly. As you rehearse, you want to move as slowly as possible. This will reduce any chances of injury and helps the actors to remember that each action needs to be calculated so that there aren't any surprises in the performance.
Repeat again and again. The more you rehearse and repeat the moves, the more comfortable the actors will be in their roles. A good rule of thumb is that for each second of fighting, there should be an hour of rehearsing.
Double-check the space. Is it safe? Make sure there is nothing that will catch the actors off-guard; this should include asking your audience not to use any flash photography.
Double-check the props. Is everything working as it should? Do what is necessary to make sure that none of the props are liable to break if they're not supposed to.
Keep the performers calm. The actors should not let their adrenaline get the best of them, which may happen when there is an audience.