Is it possible, and if so has anyone actually plumbed a manual mixing valve as a backup to a digital thermostatic valve (e.g Moen U)? I have read about the battery backup option but thought about plumbing a manual mixing valve and fitting the two mixed supplies together ahead of the shower head. I think as a measure, even adding some shutoff valves to "disable" one or the other mixing valve as needed. I love the idea of Moen U but technology advances so quickly that a fall back to tried and true is a design consideration for me.
Would this break some code? Would this effect some other dynamics that I am not thinking through? Is this just a bad idea?