SST stands for Ross Smith, Gary Tarolli and Scott Sellers; the founders of 3Dfx.
The Voodoo Graphics chipset consists of two chips. A TMU and FBI chip. The TMU is the Texture Mapping Unit and places the textures onto 'things'. The FBI (Frame Buffer Interface) combines colors, adds fog and is able to do alpha blending, dithering and stuff like that. These two chips together are the actual Voodoo Graphics chipset. Small sidenote: the 3dfx technical specs refer to TREX instead of TMU. Most people refer to TMU because 'TMU' is printed onto the packaging of the chip. TREX stands for Texture Raster Engine.
This is a diagram of the Voodoo Graphics. It speaks practically for itself. See how chips are combined and think about having two or even three TMU (TREX) chips! Three TREX chained together provide single-pass, full-speed rendering of all supported advanced texture mapping features. For example: this means you can do trilinear filtering with mipmapping in one pass. A board with 'only' two chips would need two passes and a board with one chip is not capable of doing trilinear filtering with mipmapping.