Hey Tom!

Your goal is to pick both strings at the same time. Once with the downstroke, and again on the upstroke. It sounds fuller this way.

However, it still sounds okay if you hit only the D string on the downstroke and the G string on the upstroke. It ends up sounding very similar once you get it up to speed. So I always suggest to do whatever is most comfortable.

Hope this helps!
Mike