If we examine DC case only high voltage divides 50&50
but with higher frequencies and voltage spikes with high dV/dt
there is 3 meaningful capacitances. ( Inductive parts too, not drawn )
Consisted of wire to wire and wire to groud effects.
If values C1 and C2 are not equal , voltages over isolators are not equal too.
then there is supply to ground capacitance C3.
I drew only capacitances , but inductive effect between wires and parts
make same thing to happen.
View attachment DoubleOptoCoupling.pdf
I suggest to use some HV-isolator type
or optocable between devices
I remember that cheap 4N25 has 7.5kV versions.
and then
for ex. OPI1268S
• 20KV Isolation
• 30 KV/uS dv/dt immunity
• 2 Mbit/s transfer rate
**broken link removed**
KAK