So with the whole 240/120 thing you're talking about phases. These are basically electrical currents that function like wavelengths, operating at specific frequencies. The amperage basically says how much punch that phase/frequency can have.
If you put two twenty Amp circuits on the same neutral. The electricity will pass through both motors, and come back to the panel. The panel will read that those breakers are putting out more than 20A and trip the breakers.
BUT
If you put those breakers on different phases, the wavelengths alternate. Instead of really high peaks and valleys, there are more frequently occurring similar ones. So the breakers will read with in limits, and function. This is called sharing a neutral
What your guy is trying to do, is save a lot of work, and some money on material. Instead of running two wires, he's trying to share neutrals.