22
Ok... Technically (I think :S, havent tried it personally) You could use that code in conjunction with my subsite module to run multiple sites from a single install with shared common tables...
Basically add that code to your mainfile inclusion code, make your domainA.mainfile.php and domainB.mainfile.php, make a copy of domainA's tables but rename it to be domainB (eg, you could have the prefix for domainA's database as 'domainA' and then the prefix for domainB's database 'domainB'.. BUT AT THE MOMENT THEY MUST BOTH BE IN THE SAME DATABASE!!!!) install the subsite module onto domainB and then choose which tables you want to make common.
Sounds simple doesnt it :P
Come to think of it that may not work just yet... - Consider it added to the 'wish list' for the module :)