Welcome to the City.
Sounds like you love your kids and your job. Twins are enough to stress anyone. (I've known a few people with twins.) I would start by asking what kind of support system you have? Family, friends, a mom's group? What do you enjoy away from work and without the kids? These are all important because you're going to need to balance things out.
When you say "run a rescue squad", I'm (and most on here) assuming that you mean that you're a director/supervisor for an EMS squad. That takes a lot of time and energy. If you plan on staying where you are, you're going to have to set priorities and probablly delegate work. You will find that you're not able/willing to do as much as you were BC (before children). Kids require a lot of time, energy and love. They get sick, have regular appointments, and need family time. This definately doesn't change as they get older. (My kids are 15, 12 and 11, and I think I run more now than I did when they were younger.)
If running a squad means enough to you, you can do it. It will take time from your kids. There will be times where you feel like you're torn between duty to family and duty to job. You'll probablly have times where you want to sit down and cry. Without a doubt, there will be times where you MUST be there for your kids and work be damned. You can do it but it won't be easy.
If you need someone to talk to, feel free to PM me. (I can also give you the name of a Mom's forum that you might like and get support as a mom if you like.)
Ratty