I too love these mystery threads. My thoughts too would be a link between the two (three?) circuits that are tripping due to both RCBOs going. I'm no experienced person, as you all know, I'm still in training, but surely if two RCBO devices are tripping, then the two radials are not fully split from the ring...?
When I was learning about how rings were wired, I was taught that to get even continuity values for each socket, you don't just wire them from:
board -> 1 -> 2 -> 3 -> 4 -> board
you'd more likely do
board -> 1 -> 3 -> 2 -> 4 -> board
so just wondering if there is any substance in this.
So long lead continuity tests from both RCBOs to each socket?