When we duplicate Young's double-slit experiment we have a single source, usually a laser, shining on two slits of width a separated by a distance d. The interference pattern therefore comes from the interference between the two diffraction patterns - it's a combination of the two-source pattern and the single-slit pattern.

Generally d > a, so the positions of the interference maxima (i.e., the bright spots) are still given by the equation:

d sin(q) = ml, where m is any integer.