Are you familiar with Toyota's SynergyDrive?
Toyota uses a simple 3 input/output planetary gear drive Power Split Device. The ICE is on one, about 20 HP motor/generator is on another, then the traction motor/generator and wheels are on the 3rd. This is a pretty good introduction:
http://eahart.com/prius/psd/
Off the top of my head two simultaneous equations have to be solved to regulate speed:
There are 3 gear ratios, each input/output has a different ratio.
Sum of the torque to/from each source divided by its gear ratio must equal zero.
Sum of the RPM to/from each source times its gear ratio must equal zero.
I may have that backwards, that torque times ratio and RPM divided by ratio. Then again it depends on how one expresses the gear ratios.
Lore on the development of the SynergyDrive Power Split Device says the Japanese engineers were determined to make it work, yet it took many months of software work after the first hardware before they had a drivable vehicle. And a lot of work after that before it drove nicely.
Needless to say a Prius ICE throttle is drive by wire.