From all indications reading the literature about the video card, it is designed for a single monitor, period. You have a DVI connection and an HDMI connection, but to support multiple displays, according to AMD, you need a DisplayPort adapter or DisplayPort capable monitor which would connect to one of the connections, or you need a second card linked to the first. With the proper connection adapters, two cards could support up to 6 displays, but without the adapters (which you don't mention having), only a single display is supported.
LMDE Mate 64-bit, LM17 Mate 64-bit
Debian Mate 64-bit, Xubuntu 14.10 64-bit, Ubuntu-Mate 14.04 64-bit, Antergos Xfce 64-bit, PCLinuxOS Mate 64-bit