You are right about the Ford and Toyota situation, but neither originated the single planetary two motor torque split design.
The concept was originally created by TRW (a major auto supplier) in the late sixties and described in a seminal 1971 SAE paper. It was first used for flywheel energy storage in buses before being adopted by Toyota and Ford for passenger cars.
Here is a fascinating 90+ slide 2005 seminar from JNJ Miller Design (an engineering consultant house) covering some history and providing detailed engineering performance data (including equations) for the mainstream power split designs: http://john2211.nl/Hybride_links_files/Miller_W04.pdf
Also in there is an interesting overview of the GM-Allison dual planetary gear system used on buses that became the basis of the future Volt hybrid.
