When both spouse have HSAs, if one spouse has Family coverage, then both spouses are considered to have Family coverage (even if their actual HDHP policy is Self-only).
Not only do they both have Family coverage, they also share the annual HSA contribution limit for Family HDHP coverage. That is, the aggregate of the contributions to the two HSAs cannot exceed the Family limit of $6,900. Therefore, the entire amount of $3,450 is considered to be in excess.
When TurboTax tells you that you have made excess contributions to an HSA, then the excess amount is immediately added to Other Income on line 21 on Schedule 1 (Form 1040). You are then given the option to withdraw that excess before the due date of the return. This is generally the preferable thing to do, if the money is in the HSA to withdraw; otherwise, you will be penalized on the smaller of 6% of the excess amount carried over to the next year or the value of your HSA on 12/31/2018.
So, to answer your question, in this case, the money won't be in the HSA to spend next year on qualified medical expenses.
If, however, you do not withdraw the amount before the due date of the return but spend it on medical expenses instead, then you have to carry over the excess year after year with a 6% penalty each year until you make a distribution from the HSA of the same amount as the excess, which also incurs not only normal income tax but also a 20% penalty. You see why withdrawing the excess in the first year is normally preferable.
If you withdraw the excess, be sure to do it BEFORE April 15th, and to also tell the HSA custodian that you want a "withdrawal of excess contributions" (it keeps their paperwork straight). The custodian will send you a check for that amount. Then, early next year, the custodian will send you a second 1099-SA with the earning on the excess amount - this will be added to your 2019 tax return.
