Sorry, I cant post a rule and not going to spend hours looking.
It might not a be a written rule like allot of things in Thailand, but the officers at land boarders always want to see the exit stamp from the other crossing and would never accept it being in a different passport.
My old business partner had two passports and tried this in the past and was never allowed, even if we spent a few days in Cambodia it didn't make a difference. They want to see the exit stamp in the passport and will only use the passport with the exit stamp.