I assume that the existing card is the same?
Once you install the second card, ignore windows warnings about "New hardware found" and:
1) Delete all existing Nvidia drivers and control panels etc.
2) download the latest updates from:
3) Re-boot (remember the little bridge connection that joins the 2 cards together)
4) Ignore the new hardware found bleating
%) install the drivers you downloaded in step 2.
Re-boot as asked.