Being a step-parent involves its own balance, and even with 20+ years of parenting experience already, it's a new game in a lot of ways. While starting out fresh with a brand-new baby is intimidating, so is arriving partway through in the lives of children who have been shaped without your influence (and doing pretty well before you came along, frankly). The fact that I have been a mom for so long makes me tread lighter in some respects, I think, since I try to be very conscious of not stepping on the kids' mother's toes. At the same time, it's important for my stepchildren - who are very smart, funny, and caring people - to learn to respect me, and hear my voice in our home as equal to their father's. With shared custody, there is frequent negotiation of arrangements, and while that occurs directly between the natural parents in our family, it does affect me too and my husband gets my input (whether or not he asks - but I should say he nearly always does).
I think it matters for both kids, individually as well as together, to have time with just their dad, so I'll sometimes opt out of participating in activities with them (it helps that I'm not much of a GameCube fan). However, my husband and I also want to give them a good sense of family, which is fostered by the things we all do together. I'm re-assessing boundaries all the time, and I think one of my most important roles is as sounding board and support system for my husband, who was always a pretty involved father but is even more so now. There hasn't been an assumption that I'd do certain things just because I'm "the mom" - if anything, it often goes the other way since he's the "official" parent. Especially lately, as my outside work has wanted more from me and I've been putting in some long days, it's helped to know that things on the home front can get done without me. I wouldn't say it's enough to erase the guilt totally, but it does decrease it. My new family seems to be handling things pretty well so far - we're lucky, but I know how much it matters, and that makes this balancing act especially important.
Full-time work, part-time kids...it sounds like everything has its own nice little box, but we all know it's a lot more fluid than that, which is why we struggle with balance in the first place.