There is no such thing as a "family" HSA. HSA's are owned by one single individual only, no matter what kind of coverage you have.
There is no such thing as a "single" or "family" HSA, an HSA is just a bank account. The contribution limit is controlled by the kind of insurance you have, but there is only one kind of HSA.
If you (spouse A) were covered by a family HDHP up until October 1 and you are over age 55, your maximum contribution for 2025 is $7125 from the family HDHP limit plus $833 from your catchup provision. (10/12ths of the annual limit).
Your spouse (spouse B) can open an HSA in their name right now, no need to wait for 2026. If your spouse is covered by a family HDHP for the whole year, and no other disqualifying coverage, they can contribute up to $8550, plus $1000 personal catchup if they are also 55 or older. Their contribution under the family limit is reduced by your contribution under the family limit, but their catch-up is not affected. That means that if you already contributed $7125 under the family limit, your spouse can contribute $1425 under the family limit plus $1000 catch up.
For 2026, if your spouse is still covered by a family HDHP, as you indicate, then your spouse can contribute $8750 plus $1000 catch-up (if 55 or older). If your spouse changes their coverage to single only, they can contribute up to $4400 plus $1000 if 55 or older.
