Two CPU's tend to be better than one and are mainly used in servers and high end PC's that run heavey load applications.
On servers, the load placed upon them is divided by 2 [ 2 CPUs ] meaning the server can process more information at any given time. So technicaly you have twice the speed.
In desktop PC's, the 2 CPUs are not always taken advantage of, only some applications are able to use both CPUs at the same time.
They still work on a single monitor, dual monitor systems are controlled by your graphics card rather than your CPU.
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