A skill that utilizes two characters is an interesting idea, but it could be a little unwieldy. You'd need to get permission to use it every time. In addition, I would advise against having a skill that only works with a single specific character. Instead, go for something more general. I don't know about Frog's canon abilities, but an example that fits the proposal you gave might be:

[b]Dual Tech: Dual Counter[/b] - puts a targeted ally within a few meters into a special counter state. If the Target blocks an attack within a second, the attacker is stunned while Frog will zip over and deliver a reprisal.

Aerial Blade / Leap Slash would be acceptable.