Hi! I was very interested in the subject of littermates!!!! I have two sisters not through choice, I took them to look after for six months and now have them permanently.
I experienced most of your problems and still do to a degree. I am a little sceptical of dog trainers there are good ones and bad ones and how does one tell the difference.
I would say have a little more confidence in yourself for a start you have not done so badly. I used to be terribly embarrassed by these two and thought people would be judging me on their bad behaviour.
I have had them a year now and they are improving and I am sure yours will too, patience, and understanding and observation which you are already displaying.
I had one very hyperactive one but even she is settling down a bit. I remove their toys from the sitting-room when I want them to settle and calm down, which seems to have worked.
I also ask people who call to totally ignore them until they stop their barking and once they have gone and lain down are then rewarded.
The barking at other dogs still happens but if the other dog owners are good and let the dogs meet and sort out who is top dog all is well.
I think having them neutered is the answer to a lot of the aggression when meeting other male dogs and would always advise that. They are naturally protective of their own and this has to be taken into consideration.
I have had three generations of Cairn terriers and had no problem with them. I also had Dobermans not together, but one after the other. They were both soppy and adored other dogs regardless of size, and loved the cat.
These two Spaniels are certainly different! they are still separated from our Birman. He is upstairs and they are in the sitting room when not being taken out.
I am sure if it was just one, the problem would have been solved and the fact they are littermates brings out the pack instinct.
It is all very easy to say exercise them individually but in practice is not so easy.
Friends say they see a big difference in them and they are certainly not so hyper-active, not perfect, but getting there.
I hope this gives you some hope, it will come right in the end. Do not let them know you are worried or like children they will play you up.
Dogs being very sensitive pick up on when we are nervous or sad or worried. Do let us know how you get on with them.