Fusion Flare inflicts damage. The base power of Fusion Flare will double if any Pokémon has successfully used Fusion Bolt previously in the same turn, with no intervening moves in between, except perhaps for failed moves. If this power increase applies, it also causes Fusion Flare to have a different animation than normal. Likewise, Fusion Flare will cause a later Fusion Bolt to have doubled power if it's the next successful move used in the same turn. If the user is frozen, Fusion Flare will thaw it before using the move.
If a single Pokémon manages to use both fusion moves consecutively in the same turn (via Instruct), the second one will still receive the power increase.