Intel learned from their P3 mistake. The P3 processor worked perfectly and easilly in dual mode. This made the P3 Xeon a terrible seller (it was only worth the money if you needed more than 2 processors). Since Intel has a larger profit on the Xeon line, that mistake cost Intel a lot of money.
Now the P4, while it has the same core as the Xeon, was made to work only as a single processor. It might be possible to work around it, but the added cost for some custom workaround isn't worth it. Intel also made the price difference between the P4 and the Intel Xeon much smaller than the price difference between the P3 and the P3 Xeon. Thus there is less incentive to try to make a workaround.
An Intel Xeon costs roughly $100 more than the equivalent P4. A theoretical dual P4 motherboard would cost about the same as a dual Xeon motherboard. Thus you would only end up paying $200 more to go the dual Xeon route.
The current sweet spot in Intel Xeon processors is the 1.8 GHz Xeon (Northwood core) for $232. Hopefully next months price cut will include the Intel Xeon pushing the 2.0A GHz Xeon into the $250 range. If that occurs, then a barebones dual 2.0A Xeon can be built for about $1300 (including shipping and tax). Of course a decently equipped dual 2.0A GHz Xeon will still be in the $1750 range:
Dual 1.8A Xeon: 2x$232 = $464
SuperMicro MB: $390
Cheap videocard: $25
20 GB 7200rpm: $55
128 MB RDRAM: 2x$14 = $28
Case + PS: $200
CDrom, floppy, mouse: $50
Linux Operating system: $0
Shipping+Tax: $100
Total: $1312.
How much could you afford? After Intel's next price cut, the computer priced above might be about $1150.
Edited for a more expensive case/ps.