|Just because a graphics card has two monitor outputs, it doesn't necessarily mean it supports two monitors simultaneously. Cards which support two monitors usually have two identical output ports (eg 2 x DVI ports or 2 x VGA).|
On your card it's very likely that it has a VGA & a DVI port just to give the user more connectivity choices with a single monitor.
For example, your monitor may have only a VGA port, or only a DVI port. That won't be a problem because your graphics card can accommodate either type.
But the card isn't designed to output a signal from both ports both at the same time.