Back-roll, then Recoat
After you back-brush the paint, it's time to back-roll. Don't spray an area so large that the paint dries before you can roll it out (drying times vary greatly depending on the weather). A roller with a thicker nap is better at reaching the uneven surfaces. Many pros use a roller with a 3/4-in. nap because it gets up into more of the recessed areas. And while it's tempting to roll lap siding horizontally, always roll in an up-and-down motion. It's hard to stop the paint from dripping off the bottom of the laps if you roll horizontally.
One coat may be enough if you're just freshening up the paint with a new coat of the same color. But if you're working with new siding or changing colors, a second coat will be needed. But no back-brushing or back-rolling is necessary on the final coat, so it goes fast, and the final coat hides many of the smaller brush and roller marks.