According to the specs for that model Acer it looks like Paul is correct, Acer Support: Specifications
Also came across a picture of the back of it,
with what appears to be a HDMI port between the VGA and keyboard ports. If yours is like that and Acer configured it so that both can be used, you should be able to connect one monitor via VGA and the second using a DVI to HDMI adapter or cable. If it won't work with both then you're stuck looking at a video card to get dual monitors to work.