Double curtain rods are used when you want one rod in front of another. Double brackets are used to hold both rods. You can still use finials on the ends of both rods and other accessories such as rings, holdbacks, wands and more.
You measure for a double rod the same way you measure for a single rod. Measure the window or windows and then decide if you want the rods to extend past the window. If so, decide how much you want the rod to extend. That is your rod length. Next, decide if you want finials on both rods. A popular trend is to use end caps or something similar on the back rod and use decorative finials on the front rod. Many people put sheer curtains on the back rod and something more decorative and colorful on the front. Some also like to have light blocking curtains in the front so you have many light options for your room. Also, smaller rods usually look better for the back rod.
Wood Wall Brackets
Holdbacks are convenient to use so you can pull the front curtains back to expose the window and possibly the rear curtains. You can also use rings on the front rod so the curtains are easy to move. Wands can also help with moving those front curtains.
The double brackets can have different projections. Projection is how far the rods are from the wall. So you can specify where your rods are in relation to the wall depending on what curtains you are hanging and what style you want for your room and window.