He's talking about a hybridized setup.
I had a hybrid setup for Borderlands 2. I later decided to disable it. I just wasn't getting the performance I wanted. Although it iss due to me having an older system.
Mobo: P5Q-E (only supports two cards at 8X each)
Primary ATI Card:XFX HD 6970 @900/1400
Physx Card: EVGA GTX260 Core 216
Intel G2 40gig SSD
While playing, the GTX260 was seeing only 40-60% usage during huge Physx intense scenes.
In doing my research beforehand, the 9800GT looked to be more than good enough. I only got the GTX260 because I got a great deal on it. ($30)
Here's the thread.