Watch the Two Voices pt. 1 online guitar lesson by Brad Carlton from Guitar Lab: Solo 12-Bar Blues

Whenever the bass and melody are playing notes of the same time value, it's beneficial to think of the thumb and finger(s) as playing "together" as opposed to the opposite scenario, where the fingers are "alone" (bass and melody playing opposite of each other). This lesson is an example of the "together" approach in a 1/4 note grid.