There isn't an easy way around this to have both gauges be active and accurate simultaneously Actually, even if there were, what looks like redundancy is actually a liability. If EITHER instrument should fail, the circuit may change, and then the remaining gauge is likely to be affected in some unpredictable way depending on the nature of the failure.
For some gauges, switching just the output line between a gauge and the Dynon may work well. This will only let one work at a time, but, in the event of a failure of the active gauge, a switch can flop the signal over to the other device.
This author's personal take, though, is that unless you have an exotic engine that will blow up if you're not watching some parameter (maybe old manually-controlled turbocharged engines? I'm not an expert on exotic engines), should you lose ALL engine gauges, you still have your ears and your throttle hand; the engine keeps on spinning; you're not looking at a full-blown "need to get down in any corn field" emergency. Basically, an instrument isn't critical for continued safe flight, think about whether the added complexity of making such an instrument redundant is the right choice.
