I was under the impression that the CPU/GPU were one chip?
Ja wohl, you betcha.
The EVO uses a Snapdragon 8650 -
Snapdragon 8650=Scorpion Core plus (AMD Z430 GPU plus 600 MHz DSP)
All of these multimedia super phones (Evo, Nexus One, Droid X, etc, etc, etc) use a technology called System on a Chip (SoC) - NONE of them have a separate GPU.
The CPU, GPU/DSP (for handling graphics, video, you name it), glue logic, it's all on ONE chip, in one package.
All CPU/Math/GPU/DSP combos - even if packaged separately - must be synchronized - inter-processor communication isn't "say, if you have a chance" - outputs from one component must immediately be latched up (captured) by the target chip - a common time base is required.
I dunno - maybe it's helpful to see inside and look at your phone's motherboard - here you go:
HTC Evo 4G Teardown - Page 2 - iFixit
Ask if questions - happy to help.