Strictly no (according to AMD) but I wouldn't be suprised if they are MP compatible.
AMD have to perform strict testing on the Athlon processor to certify it as a MP processor.
You can take the risk and install two Athlon XP's but if something goes wrong AMD won't take any responsibility.
<font color=purple> **Life is too short to be pissed off all the time.**</font color=purple>