For those colleges which require both parents’ finances, you should include both of their finances in the NPC. Obviously, if they are not cooperative, you may have difficulty getting accurate information about both of their finances. Self-employment and small business income and assets can also be variables that can result in NPCs being less accurate, especially if they are not detailed ones like the College Board based NPCs that ask about such things.
Most prestige private colleges require both parents’ finances (Chicago and Vanderbilt are exceptions, and Princeton only does so if the custodial parent is not remarried). Check colleges’ financial aid web sites to see if they require both parents’ finances.
See FAQ: Divorced parents, financial aid, and net price calculators for more information . However, note that the definition of custodial parent will change in the near future to the parent providing the largest share of financial support.