Did you read the post about how to get the most out of a quad?
http://www.free-dc.org/forum/showthread.php?t=13304
using the command:

sobsvc -p:5:3:0:0:0:0:1

yours would be something like
sobsvc -p:2:3:0:0 < that will fire up 2 clients 1 per core and they should get 1 WU each..

Also what OS you running windows or *nix?

Great Sticky about multi-core *nix installs
http://www.free-dc.org/forum/showthread.php?t=16953