They are not protecting their home, they are defending themselves and this behaviour demonstrates that they are fearful of new people.
Did the older dog do this before the younger one came along?
He is right in a fear stage right now, and so things will appear scary to him that didnt before, and shouldnt in the future - thats just part of a dogs growing up. It can be worsened by having a dog neutered at/before this stage (as people very frequently do!).
I would introduce them ONLY to people you trust to follow your instructions to the letter and only to people you absolutely MUST introduce them too (and when you really think about it, thats not that many people) for the time being.
I would also introduce them to people one at a time, as each dog will be fuelling the others fear/reaction.
Use a harness and leash rather than a collar, if you feel more secure having control of th eneck or head then by all means use a collar or headcollar as well and a double ended leash. Keep the leash slack, and the first goal, is to have each dog, seperately, see and take their attention away from, a strange person. In effect you want them to look and look away and take a treat from you. Nothing more.
The distance you need to achieve that is down to your dogs, so you will need to experiment a little to find that. You may need to have someone let a guest in and you be right the other side of the room, or even away down a corridor or in another room. You may need to go outside and see the guest on the pavement first, rather than in the house, or maybe in the back yard, them at one end, you and the dog at the other.
This is counter conditioning - you pair the sight of someone strange with rewards. Nothing else, dont ask the dog to do anything, dont have the guest speak or look or offer the dog anything.
Once both dogs can do that, in the house, in the yard, on the street, separately, then you can change the criteria slightly. You could move the person nearer, or have them sit down, or attempt two dogs at once - do not change more than ONE aspect of the process at any one time and if you make something quite a bit harder (sometimes bigger leaps are unavoidable), try to make everything else easier again.
Say you can now have both dogs in the back yard with a stranger 40ft away not looking at them. You think you are ready to try having that stranger just 30ft away? Then try it with one dog first.
There is more to this process in building up the counter conditioning until your dog can relax and accept strange guests straight into the home - the main thing is whilst you are working on these starting levels you must NOT overface the dogs at all. From now on, if you dont have time, if the guest wont co operate - put the dogs in another room before you allow the guest in.
It is NOT worth riskign them getting worse, and though you think they wouldnt bite, believe me - if they feel trapped and really scared the chances are they will bite - most dogs will. Unfortunately a large number of humans WILL push their luck with frightened dogs and keep on approaching when the dog is clearly saying 'go away'!