No, 8800 GTX/GTS are single GPU cards. However, there will be an "8950" model with dual G80 chips released in late April or June. Benchmarks have yet to be released, but I suspect this card will be a interesting deal since it will retail for around $600. It will have two OC'ed 8900 GTS chips with 512mb memory and should perform very well at medium - medium-high resolutions.
If you're asking whether performance is better on 2X 8800 GTX or Quad Core 79xx, the dual 8800 destroys. Quad core was never really supported in games, drivers are bad, and the design is inefficient.