I've read 4.5 Nikayas and yet I haven't found explicit doctrine on Two Truths. Very often conventional teaching is taught without any mention that it is merely conventional and that these things don't ultimately exist.
"He words things by means of what is said in the world but without grasping at it (voharati aparāmasa’’nti)."
http://www.accesstoinsight.org/tipitaka ... .than.html
"In the same way, when there is a gross acquisition of a self... it's classified just as a gross acquisition of a self. When there is a mind-made acquisition of a self... When there is a formless acquisition of a self, it's not classified either as a gross acquisition of a self or as a mind-made acquisition of a self. It's classified just as a formless acquisition of a self.
"Citta, these are the world's designations, the world's expressions, the world's ways of speaking, the world's descriptions, with which the Tathagata expresses himself but without grasping to them (voharati aparāmasa’’nti)."
http://www.accesstoinsight.org/tipitaka ... tml#fnt-10
It seems that the only the words such as "I, me, mine, self" are conventional which the Buddha uses but does not assume eternal and happy Atta. It also seems that what the Buddha meant above was that He doesn't cling too much to words.