the secondary regulator can go anywhere past the main regulator. So if you wanted two different serving psi / co2 volumes, the higher psi would come off the first 2 valves of the 3-way, and the last one would go to the secondary, which would then feed a 3rd keg, OR another X-way gas distributor. So yes, you are thinking correctly.
You could easily add that in later, start with 3 now and add more gas lines later til the fridge/freezer is full.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10