What are the financial requirements for a marriage visa for two foreigners in Thailand?

You don't need the 400k.

Be sure you have your marriage certificate stamped by your embassy and if is not in Thai or English you'll need the translation from a certified translator such as Chula University and have it legalized by the Ministry of Foreign Affairs.

There is no financial requirement for the spouse of a foreigner to get an extension based on marriage to another foreigner who legally works here (work permit, etc). You need to go to the immigration office and see what they require. They sometimes require your marriage certificate be 'certified' by your embassy and usually a letter from the employer stating they know their employee has a spouse and that they have no problem with that person getting an extension based on their employees extension.

Sadly, a person can NOT get a "marriage visa" if a foreigner is married to another foreigner. A person CAN get a "Single Entry Non-O visa based on being a trailing spouse" OR a "one year extension of stay based on being a trailing spouse".

For employment, it requires the spouse to hold a work permit, have an extension of stay, and therefore and make minimum wage (if they are not a teacher).