Anybody Seen a Simple LED "Fail-Over" Circuit ?

Critical Redundancy - One LED fails, another takes over ?
Consider traffic lights, warning lights, similar.
It's not as simple as dividing the drive current
in half because LED brightness is not strictly
linear to the current.
Searches really don't bring up much here.
Yea, there are more complex solutions ... but
what can be done with the fewest, simplest,
most robust parts ?
Not strictly Linux, but we DO sometimes wanna
drive external displays. Usenet electronics
groups ... dismal at this point.
LEDs are great, but never "forever". They DO
fail - but for some safety apps you can't
just HAVE things go black.
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