There are different approaches or strategies for developing a hybrid electric vehicle. The one you see most often is where the electric motors are used for acceleration and peak loads with the gas engine for sustained loads. Regenerative braking is often used, so that the kinetic energy is converted to electrical energy and stored in batteries (or capacitors) instread of being thrown away as heat. This approach works best for city style stop and go driving.

It seems that building a hybrid so that the electric motors are the primary drivers (perhaps as hub motors) would be possible. I guess it comes down to cost verses gain. It might be cheaper (or more reliable or efficient) to feed the power of the electric motor into the transaxle along with the power from the gas engine rather than using electric drive motors. Then again, perhaps the approaches used today are geared more for city driving than for the open road.

There are some good reference sources on hybrid electric vehicles out there. From this source

Hybrid Electric Vehicle: Not all hybrids are alike. There are many ways to combine the engine, motor/generator, and battery. Three basic hybrid configurations are the series, parallel, and split (or through-the-road) designs.

Series. The engine never directly powers the car. Instead, the engine drives the generator, and the generator can either charge the batteries or power an electric motor that
drives the wheels.

Parallel. The engine connects to the transmission, as do the batteries and the electric motor. So both the engine and the generator/motor can supply power to the wheels, switching back and forth as driving conditions vary.

Split. The engine drives one axle and the electric motor drives the other. There is no connection between the engine and the electric components except “through the road.”

The Honda Insight uses the parallel approach, while the Toyota Prius hybrid uses a combination of series and parallel.

Here's a link to Ford's Escape SEV website. It's supposed to be marketed in 2003.