It all really depends on how you "engineer" your system.
If you want to carb at differing pressures but do not want to mess with having to disconnecting and re-set the guage then yes 2 or more regs will achieve that luxury (
I might suggest you look at a low pressure in-line guage for that. They are pretty cheap compared to the high pressure reg that connect to the bottle.
As far as dispensing pressures. There are different methos for that. Many wil carbonate at a high pressure to get it done quickly and then back the pressure down for dispense.
Others, me included, calculate the temp, pressure, and line resistence in a way that will allow the pressure to stay constant but put enough resistence in the lines to minimalize foaming.
"Balancing a Direct Draft system" for details.