I'm not 100% sure but I guess it works like regular x1.2 skill rate.
I tried in my first turn team with Cybidium + 3 x1.6 skill gurls + Fennel (x1.5 skill for herself on first turn) and it was displayed like this:
- ATK boost
- ATK boost on first turn
- x1.6 skill rate for everyone
- x2 skill for everyone and x3.5 skill for Fennel turn one

If maths on this game are correct the skill rate must be:
skill rate x 1.6 x 2 so, skill rate x3.2 on turn one, since regular x1.2 skill rate applie to the base skill rate before. Or... well. I don't know the logic behind this actually. To know this exactly, we should know the skill rate formula in-game.