Seems to me just the fact that you're asking, that you have an awareness of the situation, is half the battle. Congrats on that!
I still catch myself doing this, even after 3 years. I remember the first IEP meeting we both attended for my son. We kept accidentally interrupting ea other because we were both used to being the single parent who "took care of everything." It was a wonderful realization for me - that I could "let go" because I was now with someone who was capable, responsible, and compassionate. We have very different parenting styles, but we both care and try hard.
Sometimes when one of us starts suggesting (without realizing it) how to do something related to parenting, one of us will laugh and say, "Yeah, I've actually done that in my "past life" and it worked out fine. :-)
With my kids, however, it seems a little more problematic. I guess we were just so used to be our own little family. I recognize that when my kids come to me to sign a form or ask about going somewhere or what's next on the list of chores, it's often out of respect. Their stepdad, however, feels left out, so I'm trying to be more aware of that. In my case, I think much of it is temperament/personality related. (My kids and I are all very quiet, private people, while he is very gregarious and loud.) I thank you for bringing this up, because I do need to pay more attention to the matter.
Is part of it, for you, that asking for help with anything is difficult? I was so used to having to do everything (even when married) that I still have to consciously remind myself that it's okay, and even, a positive thing in a relationship to ask for help, whether it's something little (supervise dishwashing) or big (dealing with a problematic school scenario).
Also, I'm finding that as time goes on and the kids and their stepdad are having more experiences together without me, their relationship (and thus, his involvement in many different aspects of their lives) develops naturally.
Best wishes.