When a married couple contributes to the 529 plan together, they can give a combined $30,000 (twice the $15,000 annual gift tax exclusion) together, even if the full $30,000 comes from just one of the parents. This is called "gift splitting". If the two parents consent, the full amount of the contributions made by the two parents will be treated as though one-half was made by each parent.
You may need to each file a gift tax return and the two gift tax returns should be included in the same envelope. The main exception occurs when the full amount of the joint contribution came from just one of the two parents.
The spouse must sign to provide consent on line 18 of IRS Form 709.
See the instructions for IRS Form 709 and also IRS Form 709 itself.