If possible make sure the first introduction takes place in an area that neither ferret has ever been before, not just an unused room in your home but preferably someone else's home. If this is not possible, try putting vanilla extract on their noses to confuse their smelling, and bitter apple on their necks to discourage biting. Expect mild tussling, but if one ferret starts drawing blood, separate them immediately, and try again at a later date. Remember the established ferret must not feel like they are being replaced. Do not ignore them for the newer ferret.
Ferrets love to worm their way into any little hole.
As small as 2x2 inches or smaller for kits can be very bad. Crawl around every square inch of your home to look for holes near the floor and under cabinets, especially in the kitchen and laundry area. Even holes inside cabinets should be blocked just in case. Also watch out for heaters or furnace ducts. Recliners and sofa-beds are extremely dangerous, many ferrets have gotten crushed in the levers and springs underneath.