The CPU and the GPU both work together - like having two people working rather than just one person who might be genetically enhanced to be faster.
Why do you think that the best and fastest computers are ones that have major GPU's in them?
The cheapo models have the graphics being handled by the CPU.
No, the Nexus I believe is rumored to have and OLED screen which is superior to the screen the Droid has. It probably looks better and takes less energy to run.
However as far as pure power goes there is an argument that the GPU in the Droid properly used can make up for the difference in speed between the two devices.
I don't understand your logic that they don't work together. Of course they do. They just don't live on the same chip which makes it actually a better arrangement.
Think of the google server farm. They run thousands (or hundreds of thousands) of computers running together to get us our results faster.