Firstly dual graphics is not only for multi display it supports it. Secondly you cannot crossfire two AMD cards that are of different models. Also you cannot SLI two difference NVIDIA cards of different models.
SLI = Installing two NVIDIA graphics cards in one system
crossfire = Installing two AMD graphics cards in one system
For example I had one NVIDIA gtx 560 which I used for two monitors (not used for gaming but office uses)