I also took the screamer for cranialsacral osteopathy and had four sessions- after that she gradually became a contented baby. Unfortunately she still has a really loud voice so when she cries nowadays it still jolts me back to those early days, and I get a pang of nervousness! I hope u find some of the advice here helpful. I remember feeling exactly the same as you at week 7. I was always relieved when someone else was dealing with my screamer, she used to make me feel angry and frustrated and guilty and panicky all at once! We started cranialsacral about 7 weeks in at a place in Newbury.
I think it is normal to feel frustrated with the crying/fussy/difficult baby and feel closer the the less fussy baby at times. This is one of the difficult thing about raising twins, you feel guilty for these feelings. One of my twins is much more difficult than her sister. She used to cry a lot when we were out, so much so that I go too nervous to be out alone with them. She settled down and now her sister seems to be the fussy when when we are out. I often have to cut the outing short as she is crying so much.
Speak to your GP/HV about this rather than worry about it all on your own.
I have 6 month old twin girls who were 7 weeks prem. one of them is a real screamer and I find her really difficult to deal with. She screams really loudly if I try and make her do anything she doesn't want to (eat, sleep etc) and it really panics me especially in public. I've managed to convince myself that there is something wrong with her like autism and I'm finding it really tough to show her the same affection that I show her sister so full of guilt and feeling really down as a result. What do I do??