My wife and I are partners in our company. She owns 51% but I have the controlling vote. Do everything through a lawyer. Ideally one that speaks the same language you do if you don't speak Thai. I trust my wife with everything but I'm also not stupid. If it's not a company then make sure what you have invested and what you are willing to invest, you are also willing to loose. Again, through a lawyer that speaks your language and everything translated and signed.
It will confuse them, some will then assume they are both fake.
It's just finding someone that knows the rules and how to set it up. Some people are lucky and get it all done in 30 min. It took me and my wife over 3 hours in the branch. They couldn't work out what to do with my middle name for 45 minutes. Until I got fed up and took the form and translated it with my phone and filled it out.
There is always extra 'processing' costs and every time they call it something different.
Even having all the paperwork and a print out of the relevant laws they can still say no. They try to make you worry about time so you will pay what they think is necessary to process.
Me and some other foregners here call it all the Thai way. Unfortunately they can and do, do whatever they want sometimes.