It seems that at this point the door bell or knocking is a trigger to their barking and then they feed off of each others energy and continue barking.
They are alerting the pack there is a potential intruder AND trying to scare off the intruder. Not their job. A bark or two would be okay if they stopped when you said so, but that will be tough to control when you aren't around.
This is often a symptom of their over all relationship with you, and with each other. If they aren't listening to you in general then they aren't convinced you are the leader/decision maker of the house and they are taking charge.
It is important to make a shift in your relationship in everything you do, so they start to look to you for guidance and advice instead of just independently reacting to each other and the door.
Engage them as much as you can with lots of direction throughout the day. Even the basics are handy when thats all you've got. But try to build a new vocabulary and introduce as many new skills as you can. This gives you lots of opportunities to communicate in addition to changing their minds about who is in charge of the house. The more you engage your dogs through out the day the better leader/teacher you become and the better student/followers they become.
As mentioned earlier - you can teach them to go to a spot when they hear someone at the door and that can work great. But can be challenging when you aren't around.
Another piece of the puzzle is to desensitize them to the knocking at the door. You need to have them on leashes attached to someone so the person can direct/correct the dogs when they begin to bark. But then you need to have someone repeatedly knock or ring the bell. First do it in a slow rhythm - slow rhythm is easier to adjust to. Then change up the rhythm as the dogs stop reacting to the slow rhythm and get them adjusted to a new rhythm. If they bark then you should step/stomp towards them quickly and say 'no bark' in a firm, abrupt tone, then turn away. The forward movement towards them is pressure and the turning away is release. If they don't bark you could treat them or offer calm praise. Don't over do it. Too much praise can get them right back to an energized state.
You should also randomly knock when they are eating, when you are watching TV, before you go out the door, when you come back in, etc. Remove the idea that a knocking sound always means an intruder. Make it random and exhausting to the point where they don't have the desire to bark at knocking anymore. Bore the heck out of the whole thing, for example, dogs raised in a vacuum cleaner store don't have issues with vacuums.
Are they crate trained? Sometimes dogs prefer to be in a small environment and will stay calmer over all in a crate. Less responsibility.
Put a note on the door asking people to be patient while you get your dogs under control. This takes the pressure off of you and will give you a chance to gain control before opening the door. The big mistake people make is they shuffle the dogs back from the door (as they keep yapping), and then let the person in. Failure on all levels. We must always repeat, repeat and repeat until we reach success. Even a little success is a teaching moment.
You should ask friends to come over to practice and ask them to come to the door and knock, practice having the dogs go to their spot and be quiet, open the door and greet your friends, but always be ready to correct the dogs and put them back in their spot. Or just back them down the hallway and claim your space, then open the door - always be aware of the dogs and ready to back them away from the door (leash is handy for this). Until they can remain calm while the person comes in. Oh, and be sure to have the visitor ignore the dogs until they are calm and relaxed.
Dogs will challenge you 3-5 times before they change. However if you have taught them that challenging works they will challenge you even more. So be patient and stick to your guns. Do not quit!
Because this is bigger than just barking - you have more work to do than just stopping the barking. But the more you can create structure in your everyday living the easier it will be to deal with this issue.
Love Them & Lead Them,
~Elizabeth & Doug
Dog Training the Way Nature Intended