I think you will have to check what features your graphics card has...

My Radeon has a DVI, VGA and S-video port but I can only use the DVI or VGA at any one time and the S-video. My secondary outputs at 640 by 480 anyways, not exactly useful (its for TVs). Your card may differ but since the X300 is a budget model replacing the earlier Radeon 9000 series, the features may be similar.

An easy way to tell if the ports are on the same card is to look at how they are orientated. On a tower case, ports on the video card should be horizontal and ports on the motherboard should be vertical. The other port that's further away is most likely the on board graphics. The reason why it doesn't work is that if you have a graphics card, the onboard video is usually disabled.