GF planned to implement them in RS but stopped at some point. They are different from the combo moves from Gen 5 because combined moves are used simultaneously on the same target (less prone to fail or make critical damage)
>Unused code for Move Combining
>Hidden in the code is a dummied out function that allows the player to combine multiple moves into a single, different move during double battles. The function only contains information for the move Gust and Ember, which would combine into Heat Wave.
>https://tcrf.net/Pok%C3%A9mon_Ruby_and_Sapphire#Unused_code_for_Move_Combining