Maybe depends on the particular immigration office. I had a friend coming in on a METV last winter, and got a years extension based on being over 50 directly from that in the local immigration office.
I too just carry the thai DL, and in my phone i keep my wife and close family listed as ICE (should be internationally recognized as "in case of emergency) before their name.
I think if they had to, they would not have a problem meeting the financial requirements individually.. But if they can avoid transferring 2x 800.000 to Thailand I think they would prefer that.. And as I understand from Robert Lagas answers, that is possible with the dependent extension based on extension based on over 50.
Can do ;) It was just my two cents. It does happen, but dont count on it! Maybe Buengkan being such a small, and relatively new, provinse is different than most. Been coming there for years, and know a lot of the faces, and they (most of them anyway) know both me and my father.