There's no problem connecting one additional card (as long as your PSU supports it). Just use a dummy plug - http://www.overclock.net/folding-home-guides-tutorials/384733-30-second-dummy-plug.html
to make the driver think there is a monitor attached to the additional card.
Overclocking with MSI Afterburner works good. You just need to "unlock" the software to make all overclocking options available. An example how it's done - http://forums.guru3d.com/showthread.php?t=327291
This should enable you to tweak all your cards. You can switch between them via "Settings>General tab".