Two valves in parallel working on split range signals can give very high turndown capability.
The situation that should be avoided if possible is that the larger valve operates in the “cracked open” position – one way to avoid this is to program the PLC or DCS to shut the small valve and use only the larger unit once the capacity of the small valve is exceeded.
An alternative to the two valves in parallel is to select a valve with a high rangeability such as a V-port ball valve.