For dependent tie breaker rule, does the IRS use parent's AGI income or household AGI (both parents are remarried to someone else filing joint returns w/ the step-parent)

Divorced parents both claim child as dependent, child lived with each parent equally. Tiebreaker rule says the IRS gives dependent to parent with higher AGI.  If both parents have remarried, does the IRS only look at the parent's individual AGI or the household AGI (both filed married jointly returns)?