Skirmishes: Secondary Attriubute for Soldier?

Hi guys! I hope I don't sound silly here by asking this but I got a question about the secondary attribute you get for your soldier after hitting 35.

Hows it work? does it turn my soldier into a duel class? Is their requirements in using multiple attributes?

I have tried to use this feature and can not seem to figure out how to use the secondary slot. Not sure what or how to use it.

Any suggestions?