Two graphics cards in one system? What a great way to get extra high-end graphics performance! But there was a time when such a thing wasn't just a neat (if expensive) option for top-of-the-line 3D rendering power, but it was a necessity for any 3D at all.
In the mid '90s, computer graphics were evolving quickly. From the poor, low color graphics of the original Color Graphics Adapter (CGA) things had evolved to the stellar (only by comparison) Video Graphics Array (VGA), and was evolving beyond that in a rush of standards that turned into the currently used independent SVGA standard.
The higher resolutions and colors allowed for true graphical interfaces to be profitable on PCs, and the introduction of early versions of Windows. This required more memory and processing power than standard video cards had, and led to the rise of performance video cards, or Windows accelerators. These were strictly 2D cards, but now 3D graphics were becoming increasingly more important.
It's not hard to realize that 3D graphics rendering is more computationally intensive than 2D, and reading anything at all about the techniques involved will soon show just how easy it still is to underestimate the nature of the problem. Given that complex 2D rendering had required the development of special equipment, 3D rendering was hard to achieve at all.
The solution? Design a card that did nothing but 3D rendering. 2D rendering would be handled by an ordinary card, and the specialized one would feed its output to the normal video card, which could then pass it on to the monitor.
And that is just what was released in 1996, known as Voodoo Cards, powered by 3Dfx's chip that had been powering arcade games for the previous couple of years.
Everyone quickly fell on the bandwagon. First-person shooters had been popular for the last several years, and needed 3D graphics to work. These cards opened up all new possibilities in game design and graphic performance and the cards themselves were soon essential equipment for any serious gaming system.
In 1998, 3Dfx was still riding high as the Voodoo2 was released. It was another state-of-the-art 3D graphics solution that required routing through another card for 2D work. However, it was expensive, had resolution and color limitations, and was competing in a market that was changing.
Voodoo2-based cards did well, and were preferred by some people over other solutions for the ability to match the highest-performing 3D graphics card with their favorite 2D card. However, cards that could do both 2D and 3D were getting better, and 3Dfx got increasingly squeezed out of the market.
There were further products, but these two were easily the high points of the company, and in 2000 3Dfx declared bankruptcy, and was purchased by nVidia.