My first test has not shown a significant period variation when the LDR is lit or shadowed, I wonder what is the best placement for the light-sensitive resistor. I'll have to build a few boards and test them...

OK, let's do this :-)

OK it seems I must engineer the system better. I made a 2nd circuit and they seem to naturally work at different frequencies. They appear to "read" each other when placed close but there is a weird 4:5 frequency ratio. I'm not sure if the feedback keeps the pulses in sync or prevents them, though I see they seem to lock for a cycle or two and then....

I must analyse the phase behaviour. And/or design a phase detector, or something.