My situation is slightly different in that I need two master valves and to be able to assign them by program or day, not just by zone. I irrigate greenhouses with two water sources — reverse osmosis and fertigation — and would like to be able to assign a master valve to each source so that I can select the source for each program or day. For example, a typical irrigation regime would be fertigation 6 days a week with an RO flush on Sunday. I would never need to activate two master valves at once.