there is no such thing as a joint HSA
and yes Turbotax is messed up because it assumes if you had coverage on 12/1 you are using the last month rule.
the LMR is from IRC 223(b)(8)
(8)Increase in limit for individuals becoming eligible individuals after the beginning of the year
(A)In general
For purposes of computing the limitation under paragraph (1) for any taxable year, an individual who is an eligible individual during the last month of such taxable year shall be treated—
that is not your situation so the LMR should not apply
the basic limits come from IRC 223(b)(1) and (b)(2)
which proscribe a monthly calculation of the eligible amount (from IRS Notice 2008-52)
Under §§ 223(b)(1) and (2), the maximum annual
contribution to an HSA is the sum of the
monthly contribution limits determined
separately for each month, based on eligibility and health plan coverage on the
first day of the month. For this purpose,
the monthly limit is 1/12 of the indexed
amount provided under § 223(b)(2)(A) for
self-only coverage ($2,900 for 2008) and
under § 223(b)(2)(B) for family coverage
($5,800 for 2008).
this is from IRS PUB 969 section on HSA
Last-month rule. Under the last-month rule, if you are an
eligible individual on the first day of the last month of your
tax year (December 1 for most taxpayers), you are considered
an eligible individual for the entire year. You are treated
as having the same HDHP coverage for the entire
year as you had on the first day of the last month if you
didn’t otherwise have coverage.
If contributions were made to your
HSA based on you being an eligible individual for the entire
year under the last-month rule, you must remain an eligible
individual during the testing period.
I agree with you that there is no requirement to use the LMR.
so for 1/1--7/31/20 each of you could contribute to your HSA $3650* 7/12 =$2070.83 for both in total $4141.66
for 8/1/ -12/31 since you had family coverage the balance of $2958.34 could be contributed to your HSA a/c's. this can be split evenly or any way you want. since you had family coverage during those months
there is a rule that says if either spouse has family coverage both spouses are treated as having family coverage. The contribution limit is split evenly between the spouses unless they agree on a different allocation.
a way to look at this for 2020 is that a minimum of 2070.83 had to go into both a/c's and the balance split any way you want.
in Turbotax 2021 the 8889 for each of you has section III which relates to failure to maintain HDHP when using the LMR. this should all be blank since you didn't use it. then 2021 should d not be a problem
