Does the 400,000 THB requirement for the Non O marriage visa extension need to be in my name, or can it be in a joint account with my Thai wife?

You can ask immigration if they will accept it but I'd be on the cautious side and put in an account that it's solely yours. I'm going on Monday to hopefully get my non O as my 400k has been in for 2 months now but I'm a bit worried I might have to do another two months as last week the balance dipped below 400k by Less than 100thb for about 7 hours but I'm really hoping that it won't be a problem, so make sure you don't do that as well 👍

IF your immigration office will accept you using a joint account with your wife for your proof of funds (and not all offices will do it) they require double the requirements, SO if you're getting a marriage extension using banked money method on a joint account instead of just 400K baht you need 800k baht AND if you're getting a retirement extension using banked money method on a joint account instead of the 800k baht you need 1.6 million baht :O

Check with your immigration office well before you're going for you extension to make sure they will accept a joint account AND if they won't that you have enough time to move the money so it can be seasoned the 2 months in a thai account in your name only

The 400K THB must be in a bank account in YOUR name, seasoned for TWO months. IF it's a joint account, the amount must be 800K THB, seasoned for TWO months.