This probably wont matter to the OP who only does the occasional carpet ....
But if i was getting a airflex mini , i would get a single 6.6 inside , and get them to fit a hose connector for the exhaust outlet .
Then buy the airflex booster pod and connect it to that .
So you would have very light single vac machine for doing spots or whatever u do with a single vac . And then attach the booster for full twin 6.6 in series performance .
I think i would select the 300psi pump option also ,( or a 500 if they will fit it ) the 220psi aquatec is a great pump , but lacks a bit of flow compared to a big pump . Some chap on tmf is using two of them in parallel , that probably works well , i may try that on my own express .