All AMD MP & MPX boards currently in circulation have been shown to support (read run perfectly) dual Athlon XPs. I have not read one single supportable statement to suggest otherwise.
Neither AMD, nor the mobo manufacturer will warranty or support such a configuration. Functionally the CPUs are identical, however AMD neither test, nor warranty MP functionality in non-MP chips.
It is possible in the future that a BIOS release _could_ correct this - but so far not.
-* <font color=red> !! S O L D !! </font color=red> *-
To the gentleman in the pink Tutu