The GTX 680 is capable of running four monitors on the single card there is no need to put another card. What possable reason would you want to run the 20" monitor from another card?
I have a 30" HP running at 2560x1600 and a 27" LG running at 1920x1080 off a single GTX 580. I have two other 580's but at this moment I'm testing the one card for a reason and that's the setup right now. So you do not need the 210 card in the computer to mess things up , the 680 can handle both monitors.