Hello. I have two passports. Can I go to Lao and get two Thai visa? I want to use one when I come back to Thai. Another one, after three months, when I go and back to Myanmar. Is it possible and is it legal? Thank you!

It cannot be done because you would only have a Lao visa/entry stamp in only one passport. That is something they check for and want a copy of it attached to your application.

It think you will have the same problem at any nearby location.