Is there a reason for your family member to do dual P3? For instance, some software that gets some super performance gain from dual CPUs? Has a ton of SDRAM? Already have the CPUs?
The CPUs needed would have to be either the Coppermine versions, or the Tualatin versions with 512k cache.
If your family member doesn't already have the CPUs or RAM, then it would be cost prohibitive to set up such a beast unless you find a used one on the For Sale forum. Most software would run faster on a single CPU of two GHz versus dual one GHz CPUs, all else being equal.