Just a thought but if your well pump is also 240v, do away with the safety switch and install the outdoor breaker panel. Then, use one GFCI breaker and come out of the box to a junction box(such as an outdoor receptacle box with receptacle) and run wire to the well pump then run another to the diffuser. Then, both are GFCI protected. You may be able to stay with a 20a breaker but if not, jump to a 30a GFCI breaker.
If the well pump is not 240v, someone correct me if I'm wrong but I think you could use just one leg of the 240v for the well pump and then run both legs to the diffuser.
I suggest buying a breaker panel with some expansion room too for that just in case scenario. The one we put at the diffuser had 6 slots.
I'm no electrician but I know just enough to be a hazard to myself.
