Yes, the separate secondary regulators just make this more convenient.
I’m assuming you have a ‘balanced system’, where your dispensing pressure is the same as your final carbonation pressure. So you don’t want to hit it with anything much higher while dispensing, you really just want to bring it back to where it was prior to dispensing. Otherwise, you could end up over-carbed on the low volume one.
I’ve been using some mini-kegs, and even with this small volume it’s pretty easy to draw a pint, then just ‘top off’ the CO2 to bring me back to the desired pressure. I don’t keep the CO2 connected otherwise. When the keg is mostly full, pressure drops off faster as there is little head space. As the keg empties, you’ve got more head space so more ‘reserve’ pressure in there, so the pressure changes are much less. This will be far less critical in a 5 gallon keg. After the first few, I bet you can draw quite a few pints before you see enough of a drop that you feel you need to add some CO2. It would take some time (days?) for the beer to give up its CO2 to that head space if it is only a pound or two lower. It’ll equalize somewhere in-between.
So while you may want to save some $ and space on a second regulator, I’d highly recc you add a pressure gauge to that lower pressure line, so you can easily monitor it. Bringing the pressure to where you want with the valve will be easy. If you over-shoot, just bleed it off.