As pointed out you will not really be able to achieve balanced flow to two diffusers via a splitter. The CO2 will take the path of least resistance and one CO2 diffuser will dominate the flow.up to now I optimised the flow and injecting CO2 from a splitter to two inline diffusers before the spray bars
You need to split the flow before two needle valves so you can balance the flow to each diffuser individually, as below. Two bubble counters as well so you can see the rate to each diffuser.