This short video shows you the 2 Billboard locations and how to get to them in a single match.
The first graffiti covered is located near Salty Springs. Go to the map location shown below.
Once you visited this billboard you must head to Shifty Shafts where the second billboard is located.
You can travel to it with a Quad or hoverboard. Both are located near the first location, in the path to the second location.