Та Туяаг яаж мэддэг вэ? - "How do you know Tuya?" This is a general question, no other meaning or nuances included. Та Туяаг яаж мэддэг юм бэ? - the translation would be more like "How come you know Tuya?" I would guess the person who is asking somehow, didn't think that the other person might know Tuya, or was surprised to find out that he/she knows Tuya. In any case there should be some previous conversation before this question.