The words “τάχατες” and “τάχα” come from the same root and they are synonyms. When they are used in a sentence they express question, doubt and uncertainty. “τάχατες” is more informal than “τάχα”.Τι τάχατες ζητάει;
Τι τάχα ζητάει;
All these phrases mean the same thing :“What is he asking for?”
The first two of them show much more doubt and uncertainty than the third.