There are several posts similar to this one.....(I did not link whole thread) .....
chesterubi1
68
Posted November 16, 2020
"I went to Chiang Mai Immigration today and can confirm that you can use a Joint Bank Account, for applying for a 1 year extension based on Marriage, as long as you have at least double the normally required funds. I went to the bank and got the necessary documents and have submitted my application without issue....,"
As for your other point that is incorrect.
You can have a signatory on your bank account and there signature appears only under ultra blue light.
It's not a joint account.