dual monitor setup on my video card with 2 video ports

[Ubbu]

I received a computer with a video card that looks like it can accept
2 monitors. If this is what I think it means, I can and want to have a
dual-monitor setup.

Currently, only one (the left one) DVI port works. When I unplug the
monitor from the left DVI port and plug it into the right DVI port, my
monitor stays blank/black. Why is this?

 Is it possible to have a dual monitor setup with my computer, so that
I could have, in effect, the screen space of the sum of the 2
monitors? Does this make sense?

If so, how do i set up my ubuntu to do that?

 You are probably wondering what video card is in my computer.I did
lspci, and here are what I think are the pertinent results from lspci:

 	
05:00.0 VGA compatible controller: ATI Technologies Inc R480 [Radeon
X850XT Platinum (PCIE)]
 05:00.1 Display controller: ATI Technologies Inc R480 [Radeon X850XT
Platinum (PCIE)] (Secondary)


 Please note: I am not sure if the description in brackets (i.e.
"Radeon X850XT Platinum (PCIE") is what the computer found by itself,
or whether that text was something I typed in manually. I do not
remember now. So if the text in brackets could be user-edited, it may
be inaccurate. Just a warning. I guess what I'm trying to say is that
I'm not sure exactly what video card is in my box. Please explain.

I don't know why I get "duplicate" information. Is it an error, or is
that what happens when you have a video card that can accept 2
monitors. Please explain.

 My video card has one S-Video port and 2 DVI ports.



Thank you.