I didn't have a problem when I only had William, but Louis is very different and very alert to the post coming through the letterbox and people ringing the doorbell or knocking the door. Once he starts barking William joins in.
I've taught them that they can bark for however long it takes me to walk into the hall from wherever I am, but then they have to go into the study, which is off the hall. Once they're in the study they stop barking and I can open the front door in peace. It helps that I have a babygate across the hall a couple of metres away from the front door. I've noticed that if the gate isn't there and Louis can get right up to the front door he becomes quite worried and his barking is louder.
Even if they were both perfectly behaved I wouldn't let them greet strangers at the front door. The changes to the law a couple of years ago mean that even a bit of excited barking or a friendly jump up could mean a complaint if the person at the door felt threatened. Also, much as I love my dogs why should I expect visitors, delivery drivers or the postman to like them?