This is an instant skill but it is for 2 hits. You have 1 hit followed by another. The term "casting bar" is inappropriate for this skill and should be called a "duration bar" instead as it marks the 1 second between the two hits..
The problem with Duration Bar is that channeled skills also have this Duration Bar. Channeled skills break when stunned, silenced, or the character moves, while Fighting Spirit Combination never breaks. It can be cast on the go and supplies 2 hits regardless of where the target is in relation to you on the second hit. This skill should function in the same way that "Double Chop" also does 2 hits without any kind of casting/duration bar. Casting/Duration bars use the same "Casting Bar" element in the game. The implementation of continuous casting should not be effecting this skill at all. By Casting Bar, I was not referring to the skill having an actual casting time on it, but merely using the "Casting Bar Element" in the game when it quite frankly should not be.