If I am using an AT-RT with a rotary gun.

Which do I get to do:

I roll 5 Black dice only for the rotary or two white dice for the rifle (meaning I have to choose)

Or do I get to roll the 5 Black dice for the rotary and the two white dice for my drivers rifle together equaling 7 dice?

Each mini in a unit can select one weapon to use. An AT-RT unit consists of one mini, so you can only use one weapon at a time.

It's as @nashjaee said. The only way you can use the 2 weapons in the same attack with the same mini is if that mini has the Arselan X keyword (like the AT-ST or the T-47 Airspeeder).