"Dual", not duel.
A "Dual G5" is a Mac with two G5 processors on it's motherboard -- two actual seperate G5 CPU chips. There is not a different kind of CPU chip called a "Dual G5"
An iMac G5 cannot accept a second processor; it has a single-CPU motherboard.
On the other hand, if you have an iMac and a Dual G5 Mac, you can certainly network them together.