Do both partners need $24k for a retirement visa in Thailand?

2 lots of 800,000 "baht".

There are a number of visas referred to as a retirement visa. The most common is the Non-O based on being over 50, while it might be possible for one to get this visa and the other to get a Non-O based on being their dependent it's not very common but you can definitely ask your embassy. Another alternative is the Non-OA visa, it also requires insurance and police check etc, but it is easier to get a Non-O dependant visa using this option. You can only get this visa in your home country

Yes both, however if one is over 50 and the other isnt . .they can apply for dependent visa.

The other route if only one can do it now the other can apply for a Non-O A visa which has income requirements this will help with long term stays such as educational visa help save or build to DTV.

There are many routes to do this if you do not have combined total of 1.6 million baht right now.

2 visas = 2 x $24k required.

Unless your Thai embassy will sell one of you a non-O based on retirement and sell the other one a trailing spouse non-O visa, you'll likely both need the money in the bank. Most embassies stopped selling the trailing spouse visa a while ago, and it's not available in Thailand.