The purpose of a buffer tank is to provide a high flow feed to the valve removing the restriction that the cylinder valve has. If you are using two standard cylinders then that restriction is not removed.

Typically the refillable cylinder feeds the buffer tank pressurising the system with a low flow rate. The high flow firing valve then has a feed of relatively unrestricted gas.

Without the buffer tank you are limited to the flow rate of the bottle valve and also have to consider the expansion rate of the liquid CO2 to gas. This would typically result in liquid CO2 being drawn through and freezing the system up.

Although two main cylinders provide twice the flow, a buffer tank can provide a much higher flow than this through a large port.