Thanks to all of you for the responses. The fans are 120v. I just assumed that there were diodes available for line voltage - they make hydraulic check valves for all sorts of pressures.
Looks like I need to find a different switch. I bought the switch a couple of years ago for this (future) project and suddenly when I got the meter out to check the terminals, I realised that both fans were going to run on both fan settings. I'll look into the switches mentioned and see how I can package them (the hole is already made - duh!). Thanks again.