That would probably be the connection off the motherboard and more than likey is turned off in bios. Probably should wait for someone with more experience on this. You could go in and turn it on but I'm not sure how that would work with your video card. Talking quality wise here.
You might do better just upgrading to a card that has dual monitor output.
Can you take the card from the old computer and swap it out with the one in the new computer?