For 2020, and assuming no one has some type of "other" coverage that would disqualify them, the contribution limit for family HDHP coverage is $7100, plus $1000 catch-up contribution for each spouse age 55 or over. So your overall limit is $9100, BUT the $1000 catch-up can only be contributed to an account in that person's name. You can divide the other $7100 any way you like.
Allowed
$8100 in spouse A's account and $1000 in spouse B's account.
$4650 in spouse A's account and $4650 in spouse B's account.
$2000 in spouse A's account and $7100 in spouse B's account.
Not allowed
$9100 in spouse A's account and nothing in spouse B's account.
