I have never used multiple displays on my PC so I didn't realize I had this problem but, today, I decided to use one of my old monitors that was collecting dust in my room. I plugged it in to the VGA socket in my motherboard, the power came on, but it didn't display anything. I plugged in my primary monitor (normally connected to my AMD R9-280X) to the DVI port on my motherboard to test it; that didn't work either. Could there be a wire that I forgot to plug in? Or anything else that could be causing this problem?
You need to go into the BIOS and turn the on-board graphics on. If you have a R9-280x, you would be much better off running up to four monitors directly from the R9-280x. You may need active adapters to get 3 or four running, but you can definitely run two effortlessly on your 280x. I run 4 monitors and a TV with 2 active adapters off an old 6950! I used to run 3 monitors with both a 4850 and 790GX MB, and it worked, but I did run into some weird problems with some games.