Well obviously laws are different state to state, but it sounds like your father and mother had joint custody, basically she was the legal parent but he saw you kids every other weekend or what not and paid support. That is a standard joint custody agreement as far as I know and what I have dealt with. Now by splitting time 50/50 it sounds like your father and step mom are doing a shared parenting agreement, and if so it should be worded as such, this should eliminate any and all child support obligations because as you said each parent will be supporting the child half the time. That being said there should be something in the papers that allows for who provides insurance and how medical/school expenses are split.
If it is worded as joint custody, but your father has an alternative visitation schedule that gives him more time with the kids, then chances are he will have to pay support to the residential parent, your step mom.
Something else to consider, in most cases where child support it paid it is the custodial parents obligation to send clothing ect. when the children are with the other parent, provide half the transportation, and pay all school fees and copays for health care. While the non-custodial parent basically just provides insurance, pays child support, and feeds the children when they are with them along with basics such as toiletries and a place to sleep.
Courts will also take into account the amount of time kids are with each parent and the income of both parents if it is brought to their attention. I have custody of my middle 2 kids, but they do see their father more often than the court papers specify and he pays for a lot of their activities and provides clothes ect. for them when they are there, this was explained during our divorce and he pays less support because of it. We also only used 1 attorney and basically had the papers specifically drafted to meet our needs because it was a very agreeable process for us.