<< so it might work and it might not? the tyan says 'up to'.... I'm learing to hate those words >>
Socket A IS Socket 462, they are one and the same under different names.
AMD and Tyan only officially guarantee that the AthlonMP will operate reliably in a dual processor system, though it has long since been proved that the Thunderbird is definitely capable of it.
It will not work "as well" or "as fast" as the AthlonMP would in a dual processor setup, but it will work.
I'm not sure who informed you as to the differences between the Thunderbird and the AthlonMP but they were entirely incorrect.
The AthlonMP is certified for Dual Processor operation, has an increased number of translation lookaside buffer in L1 cache. Lower power consumption and is cooler running. Has hardware data prefatching, and supports SSE instructions. It also has an on-die thermal diode for more accurate temperature readings.