Piece of cake! If you have a snout catcher/pig catcher (whatever they're called) simply have one person harness the boars snout and keep tension on it at all times while the second person does the business on the other end. As long as you are keeping pressure on the snout the boar will be more worried about his snout then whats taking place at the other end of him. If he takes a step forward just take a step backwards and keep that tension on his snout. He'll freeze up when you pull on his snout, giving the second person adequate time to do the castration.
Warning: Please wear ear plugs. If you have room wear 3 or 4 pairs.