You are doing brilliantly! Even if you don't think you are.
My DT's are now nearly 9 and the first year of their life is a bit of a blur because it was so non stop. I still feel bitter when i see people post pictures saying they could could snuggle their baby all day. I'd have loved to have done that but there was never really time as there was another baby to cuddle, feed, change, wind, settle etc.
Now that they are older, once a month I take each of them out to do something just the two of us, normally swimming or lunch and shopping so they both get some one on one time. (pre covid of course, now we do nails or bake, play a game etc).
You will always feel guilty about something. It's just the nature of being a mother. But they will be fine even if they have to wait for a cuddle or a bottle or anything else. I think being a twin has taught my two patience that quite frankly I don't really have. 
Once they get a bit older they also have a built in friend. It's been so lovely to see them thrive together and find common ground even with vastly different interests. Though since lockdown and them being around each other more (they are in different classes at school) they have become more "twin" like and have starting saying the same thing at the same time which is a bit odd as they never normally did that.
Are there any twin clubs near you? There was one when I had mine but they closed shortly afterwards (along with the sure start centre that was my only trip out each week, cheers budget cuts!).
I also have a FB group with some twin mums (all our kids are the same age) which is helpful for ranting in. They are from all over the world so there is normally someone around to reply when it's 4am our time.
So you are not being unreasonable! Hugs x