For married couples filing separately, the annual dependent care FSA contribution limit is $2,500 for each spouse. If you (or your spouse) contributed $5,000 to your Dependent Care FSA and you choose to file separately, then the excess $2,500 contribution will be added back to your taxable income on your (or your spouse's) tax return. Or, as you say "whoever W-2 it is". You cannot transfer some or all of the excess to a spouse for them to use.
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