Once the CO2 enters the tubing they will be at the same pressure, adding another inlet further on piping will only give the CO2 one extra journey in the pipework to the reactor, whereas the venturi has the CO2 constantly circulating until it is all dissolved, so I don't think an extra inlet will make much if any difference, plus more joints more chances of leaks and cost.
connectors for the CO2 tubing is cheap and easy one four way junction, one tee will be all that I need and the bespoke venturi
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