Does each spouse need separate funds for the retirement visa in Thailand?

there are several ways to go about it

Get a year-long, multi-entry Non-O-A visa from a thai consulate in your country before you come here. That one requires proof of financials, a medical certificate and a police back ground check. This visa is good for unlimited entries to thailand for the duration of the visa (one year from the date it's issued) with each entry getting you stamped in for a complete calendar year.

Get a 90 day Non-O (notice no A after it) visa before you come here and then apply for a yearly extension of stay based on being over 50 (retirement) at the immigration office here in thailand. That requires proof of finances.

You could also come here on a 30 day visa exempt entry or a 60 day tourist visa and then apply for the 90 day Non-O visa and the year extension at the immigration office too if you wanted to do it that way.

Of the choices I offered I'd say the most valuable visa (in terms of longevity, ease of use, etc) would be the year-long, multi-entry Non-Immigrant Type O-A (Long Stay) visa. (y) It's about 200USD but worth every penny.