Sure, one can be 98 PSI and the other 101 PSI. The key is they are constant pressure. And once you have constant pressure you can use the needle valves to turn the costant pressure into a constant flow rate. The other key is to pull enough CO2 out of the system to drop the third regulator inlet pressure much lower, lets say 40 PSI. Now when the third regulator poens the flow through the needle valves will depend on where they are set.
What do you mean by constant pressure? Are you saying that the neddle valves will have a large pressure drop across them that the pressure will equalise at some value much less that the ~100 psi that the tank regulators are set approximately at, and therefore both regs will allow gas into the system independent of the others set pressure?