When you go over the combined $5000 limit, the excess amount will simply be taxed as ordinary income. Per the IRS:
If the combined dependent care FSA contributions elected by the spouses exceed the applicable contribution limit for a married couple, contributions for one or both of the spouses may be reduced for the remaining portion of the tax year in order to avoid exceeding the applicable contribution limit. To the extent that the combined contributions to the dependent care FSAs of the married couple exceed the applicable contribution limit, the amount of excess contributions will be includable in the spouses’ gross income as provided in section 129(a)(2)(B).
**Answers are correct to the best of my ability but do not constitute tax or legal advice.