Basically, multitexturing combines the alpha channel of two different textures, creating more variety in the particles and making it more realistic. It was introduced in Cataclysm but widely used since WoD for fire effects.
(http://i.imgur.com/OdZnL7h.gif)
Usually, the textures used for particles in WoD are:
World\Expansion05\Doodads\IronHorde\FIRE_2X2_SHARP_MOD4X.BLP
World\Expansion05\Doodads\IronHorde\Fire_Bright_Mod2x_A.BLP
I tried to emulate this multitexturing effect in WotLK by combining both textures with Photoshop and increasing the rows and columns to give it more variety (since we can't animate the particle texture in Wotlk, each block has a different texture position). The result is quite decent. I would say it's 80% accurate because some features, like combining blend and additive blendingtypes on the same particle are not available in WotLK.
I made two texture versions: one with 16 blocks and another with 64 (more realistic). There is also a test model included.
Download (http://www.mediafire.com/file/mpmmcvq9g8s7qij/MultiTexture_Fire_Particle.rar)