the last month rule affects 2 tax years. if you used it in 2019 to qualify for an HSA contribution then you have to be covered by an HDHP for the entire testing period in 2020. you must remain an eligible individual (covered by HDHP) for the next 12 months, so through December 1st of the following year. If you fail to remain an eligible individual (change insurance plans, lose insurance plan, receive other health coverage which includes medicare) during that time, any “excess” contributions you made as a result of using the Last Month Rule will be taxed and penalized.
it does not appear you used the LMR in 2019 since you say you made only a pro-rata contribution - 10/12 of the full-year amount - the months covered by a HDHP. using the LMR would have allowed both of you to make a full-year contribution for 2019.
so if the LMR was not used in 2019, your maximum contributions for 2020 would be
for her $3550 *6/12 for her (the fraction of the year not covered by medicare) and for you $3550 *10/12 total is $4733 which can be split any way you want if you both have HSA accounts. in addition, if you each have an HSA a/c she can make an additional contribution to her a/c only for $1000 * 6/12 and for you if you have your own HSA a/c $1,000 * 10/12.
the extra amount is for being over 55
