Do both partners need to meet the 800K baht requirement for a retirement visa in Thailand, or can one pension cover both?

Recommend the 800K method. If there is a mistake in just one of your monthly transfers and the transfer is NOT documented as an international deposit, you will need to scramble to resend another 65K, to try and get the deposit listed as coming from outside of TL. If you just put 800K in an account, the potential problems with xfers is alleviated.

If each of you are on the retirement visa you each have to meet the financial requirements. The non-pensioner however can get a trailing spouse visa

You can't certify it. The only people who can certify it is the US Embassy, and they stopped doing this years ago.

You MUST make your first extension with 800,000b in the bank with an account in your name only. Your spouse would need to do the same.

It's possible one of you could get the visa and the other could get a "trailing visa" based on being married to someone on a retirement visa (this only applies if you are husband and wife as Thailand does not recognize same sex marriage).

Then during this year, you would both transfer 65,000b from abroad into your account EACH MONTH. This must show that it came from outside of the country, and it must be every single month for 12 months. If you are even a day late, you just lost the entire thing and will have to try again next year.

Then after these 12 months you can try for the monthly option.