So sorry, you two @cedarlakeinstruments and @gilshultz if I was nebulous - yes, the idea with this redundant (in the positive sense of having a backup, rather than in the sense of being superfluous) controller is that either both arduinos are on (i.e. only when they receive power from their respective USB hosts), or a single one of them is on - the idea being, that neither knows nor cares who is active; but WHEN either (or both) are active, they will each (independently) work... if that makes sense.
Sooo, with a couple of 1N4001 in series - between resistor and LED, I thought? - I should be sweet, and able to avoid the two arduinos powering each other parasitically?
I re-thought what I want to / need to achieve, and controlling the LEDs programmatically is not necessary (but, a common dimmer for all LEDs together so that the controller as a whole can be adapted to a low-light environment), so I am looking at using an external PWM controller with potentiometer such as this one: https://www.aliexpress.com/item/1005002348914350.html?spm=a2g0o.order_list.order_list_main.16.7c0f180201bPmg
Do you reckon I would be able to PWM Dim the LEDs this way, but power them from each/both VCC / 5V outs?, such as in the attached updated diagram?
And if all the switches are connected as input_pullup, that should mean that whether they are open or not, there should not be parasitic draw - or would I need 1N4001's for each used input pin also?