I'm fairly certian that OSX manages all (single, dual and soon beyond <que spooky music>...) processor use in OSX, the single meter is representative of the 'total' load (DP4 has the same 'single meter' Logic does). In os9 the individual software application needed to do this using it's own methods in order to take any advantage of the 2nd processor, hence 2 meters. To test this, open the Process Meter in the Utilities folder (top, for you unix geeks) with a nice heavy load on the machine (and i do mean heavy) and you'll commonly see processes using > 100% .. that's more then one processor right there, no muss no fuss. I'm hardly a coder, but my understanding is that Apple has done far more work then the developers (of any application) need to do to make their application most efficiently use 2 processors. .
Using a 1.25 MDD, I've found myself hitting the wall with bus throughput or disk i/o *way* before the processors could ever max out. G5 machines will change this for the better as the system bus is increased ~6-7x (167mhz to 1ghz), but I'd bet that disk i/o and system bus will remain the limiting factors for a while to come....
--hope this helps