It does when the two CPUs are in the same socket.
It doesn't support multiple sockets. It's a licensing issue, not a code issue. All versions of XP are SMP-capable, but XP Home only runs SMP with dual-core single socket chips.
That's not how the industry uses the acronym....
http://en.wikipedia.org/wiki/Symmetric_multiprocessing
"The most popular entry-level SMP systems use the x86 instruction set architecture and are based on Intels Xeon, Pentium D and Core Duo processors or AMDs Athlon64 X2 or Opteron 200 series processors."