Yes, you can control multiple Audio Boxes using a single 18R2.
All Audio Box devices that are being used must be synced to the same 18R2 controller. All of the Audio Boxes will reside on the same channel, which is channel 99. For information on checking signal strength check out Audio Box Checking Signal Strength.
Each Audio Box can play the same or different audio tracks. To ensure that multiple Audio Box devices plat the required audio please see Audio Box Creating and Formatting the MP3 Soundtrack.