This is normal behaviour for dogs -- because obviously they are having fun. The issue is that they are not doing what you'd like. But dogs that have had no training, and dogs that are not being managed, will not know what it is you'd prefer or that it is rewarding for them to make that 'right' choice. What time and training have you put into them?
And more to the immediate point (and the easiest point of intervention!) -- how/why can two dogs have so much freedom that they can do all this without anyone noticing or intervening before they destroy so many things??
Are they just left to run around the house and do as they want? How/why do they get the time and access to destroy whole duvets and other items? Also: this is seriously risky for their health, on top of being frustrating for you -- if they swallow bits of what they are chewing, they could easily die from an obstruction!
I'd suggest buying The Good Little Dog Book by Dr ian Dunbar as it will give loads of advice on how you get your dogs to be the good canine citizens you'd like them to be (as yes, this is YOUR job to teach and manage them, not something they just know to do! That's always the tough bit -- we all have to put in the time to get the dogs we want.
It is just the same as children -- if we let kids run around and do as they want with no supervision, and have not taught them self control -- then they do not know how to behave).
Also: are these two dogs that you got together as puppies? If so, that is likely an additional source of your problem right there -- and this is going to be a serious task as this is why trainers and breeders tend to say never, ever get two puppies at once. They needed to be trained and managed separately with only occasional time spent together, from the very day you brought them home. If you did not do this, and did not focus on daily training independently, then they have bonded more with each other than to you. They learn to pay little attention to the humans in the house. It may be that the solution is going to be rehoming one of the pair -- a tough option, but one that may be better for the situation, for the dogs, and for you. But you will still have a major task with the remaining dog --and seriously must limit what your dogs have access to as eating clothes like they have been means one is very likely to end up in an emergency situation as is. They just cannot be allowed to access all this stuff -- clothes, shoes, furnishings. It sounds like they are given more freedom than they have learned to to manage on their own, and have very little self control. That is a different issue from simply walking the dog til it is tired. Also, how much daily training practice do you do? Daily training tires a dog -- and gets the results you wants-- far more effectively than exercise alone.
In short: you have a pretty serious problem on your hands and you will need to deal with it seriously, as I think you recognise :flwr: . I'd really recommend looking up a nearby APDT qualified trainer and get some formal help, as doing this on your own is going to be very difficult and given that they are this old and running around like this, they will need a LOT of work and time.
But get the Dunbar book and also please read ALL the articles here too:
http://board.cavaliertalk.com/showthread.php?t=25333
And on managing two dogs the same age and why you are having such problems now:
http://board.cavaliertalk.com/showthread.php?t=10549
And decide on a management technique -- these dogs should be babygated in the kitchen or a safe playroom without access, ever, to items you do not want chewed. This is true for ALL dogs -- never assume a dog will NOT get into what it can access. Think of how you'd toddler proof a house and what you'd let a toddler do without supervision =- that is basically how anyone should approach a dog too.