I have always believed the two sizes of hoses is just to prevent connecting it incorrectly. You shouldn't get cavitation as long as there are no big restrictions in the inlet line. The head loss on the inlet and outlet should be identical, unless you have a CO2 reactor, or something else in one of the two lines, or several elbows in one line and not the other.
It looks to me like this idea will work if you use a big enough large diameter pipe so the net cross section area is a bit bigger than the cross section area of the smaller pipe. I think you need a bit larger area because of the larger surface area of the inside of the bigger pipe, which increases head loss a little bit.
I really don't see any advantage to doing this though. Two small diameter tubes are easier to hide than one big diameter tube, I think.