Do I need to increase my Thai bank account balance to 800,000 baht before applying for a marriage visa as a trailing spouse?

You may wish to top up your account from 400k to 800k at least two monts before your current extension expires as insurance in case your switch to a trailing spouse is rejected for some unforeseen reason.

For the marriage visa / extension does not one party need to be Thai?

You won't be applying for ANY visas 🙁 you both have year extensions <- which are NOT visas at all and you're not getting a new visa, you're changing the REASON for your extension (from retirement to trailing spouse)

Until you actually "change the reason" for your extension from retirement to trailing spouse you need to meet the seasoning requirements for the extension you're on <- which is not let the balance in your account go below 400K baht until you apply for the new extension.

You do NOT need to get the funds up to 800k baht 2 months before you change the reason because you're not applying for an extension based on retirement but changing the reason to having an extension based on marriage to a foreigner on a retirement extension