Don't buy one!!
I don't use it and have never used it. It's in the other van, Matt uses it all the time and I have two lads working from it next week.
I have 35 LPM on mine and I very rarely split the flow, for two operators.
Jumping from 21 LPM up to 40 LPM sounds phenomenal but in truth with two operators it is only the same as you have now.
I would buy two Zetas @ 30 LPM and if you ever need proper grunt then join the two together for 60 LPM,
Must be cheaper to go that way as well.
I am probably going to be buying a new van over the winter and this is the way I am going.
Unless you have worked from a much bigger machine and then had to split the flow it is difficult to make you see the reasoning.
Rob