Because Spouse A and Spouse B are both covered under a family plan, they are each considered eligible individuals for HSA contributions, which means that they are both able to contribute to their HSAs. The 2019 family contribution limit of $7,000 must be split between the two of them. This $7,000 limit can be split between the two spouses in any way that they choose.
However, if Spouse A's employer contributes $500 to A's HSA, that means that the couple can only contribute $6,500 of their own money to the HSAs. As a result, neither spouse will be able to claim a full $7,000 HSA deduction, because the employer has contributed $500 to an HSA on your behalf with tax-free money.
The couple can split that $6,500 limit up between the two HSA accounts any way they want. They may contribute the full $6,500 to A's HSA (with A taking the $6,500 deduction), or they could send $6,500 to B's HSA (with B taking the $6,500 deduction), or they could send some to each, with each spouse taking a partial deduction on his or her tax return.
The contribution limits and deductions for the couple are the same whether they file jointly or separately, but when they file separately they need to coordinate and make sure that they don’t go over the combined limit on the two returns.
From IRS Publication 969, Contributions to an HSA:
Rules for married people. If either spouse has family HDHP coverage, both spouses are treated as having
family HDHP coverage. If each spouse has family coverage under a separate plan, the contribution limit for 2017
is $6,750. You must reduce the limit on contributions, before taking into account any additional contributions, by
the amount contributed to both spouses' Archer MSAs. After that reduction, the contribution limit is split equally between the spouses unless you agree on a different division.
The rules for married people apply only if both
spouses are eligible individuals....
Employer contributions. You must reduce the
amount you, or any other person, can contribute to your
HSA by the amount of any contributions made by your
employer that are excludable from your income. This includes amounts contributed to your account by your employer through a cafeteria plan.
One more thing: I’m not sure why you are asking about 2019 limits, but to be clear, for the year that we are in right now, 2018, the family HSA contribution limit is $6,900. This is the limit for contributions you can make this year and is the limit you will use when submitting your next tax return early next year. The $7,000 limit is for contributions made in 2019, and is the limit you will use on your 2019 tax return, which is submitted in early 2020.