Do my husband and I both need to maintain 800,000 Baht each for a retirement visa in Thailand or can we use a joint account?

If you apply using money in a joint account your going to need double the money in any case. Thailand views joint accounts as being only 50% yours. If the money is in an account solely in the name of the applicant its possible for them to get a visa based on 800k baht and depending where you apply the other person "might" be able to get a dependent visa for no additional cost, but this is difficult. If one were to apply for the Non-OA visa it's a bit easier but still not guaranteed

You can’t use a joint account. You each need your own account with 800,000 in it.

Check the Thai embassy website too, it should have the info there.

Speaking to the Non-O w/retirement extension option...if each of you want your own Non-O visas/extensions, then each of you will need individual accounts. If you want to come in as a dependent spouse under your husband's Non-O, then only he needs an individual account. You of course can get your own account, but for Non-O visa purposes doing the primary/dependent spouse option, only one individual (not joint) account is required. And what Mel said above concerning the risk of the primary/dependent is definitely worth taking into consideration.

Even if they offer the dependent visa, you might want to apply on your own so that if one passes, the other is still eligible on their own to stay with no extra steps. Or maybe the dependent gets grace period to sort out things?? But you might not be in the frame of mind to deal with visas in that event so better to just be here on your own visa.

800K Thb each and you need to maintain it for 2 months before and three months after extension and then never let it go below 400K for the rest of the 12 months

You each need 800k.

You’ll both need the 800k there not issuing depend much anymore

Go to the Thai embassy website in Your country for the best information.