splitting them would be so hard, how will they ever get to be together because it would be like son goes to mom's while daughter goes to dad's so they are always separated and won't grow up together as siblings.
The best situations I have seen work is where each parent has the kids equally and works out a schedule where they are at Mom's 3 1/2 days a week and at Dad's 3 1/2 days a week even through holidays. If there are special vacations or trips they work those out in advance.
If one parent decides to move farther away where this isn't possible than the one moving away needs to give up their part of the equal custody agreement then work out other arrangements like every other weekend or what works as far as travel without the kids missing school. The kids should be #1 over all personal decisions.
Try to get along in front of the kids. Work out battles over the phone out of earshot of the kids or have meetings to discuss things and hire a babysitter or have the kids go to grandparents while you have these discussions.
Divorce is never easy on anyone but the kids need each other and try to show each other respect for the kids sake.