Can I double-feed a low voltage DC system?
I parallel power supplies all the time. The only problems I have found is with switching supplies sometimes they don't work well together but this is on loads that are inductive in nature. If I want to be 100% sure it will work I use a transformer based regulated supply set at the same voltage. So both supplies under normal load are being used. Setting 1 supply at a different level has the higher voltage one doing all the work and possibly failing earlier than if both are set at the same level. No diodes or anything else is needed. It is much like hooking 2 batteries in parallel for more amperage.