A bit more precise

Chapter 3:
-Go to Guadosalam and meet Mae'chen in Leblanc-Underground.

Chapter 3:
-After defeating Ixion, keep pressing ACTION-button when Yuna complains that she's all alone so a Tidus-Ghost leads her back up to Sub-Bevelle.

Chapter 5:
-After defeating Vegnagun, wait for the cutscene where YRP march through the farplane. Press ACTION and answer "Yes". If you haven't completed the above requirements, you will not be presented with that option. After the credits, Yuna can finally reunite with Tidus. If you have completed the game 100%, there will be an additional cutscene of Yuna and Tidus in Zanarkand Ruins.