I get an answer from Shelly
Dear Sir/Madam,
Remember that Shelly EM supports one phase measurements. Whatever is attached on it’s L channel is taking the voltage from there, the rest of the channels, the phase is shared between them, so the clamps will be using this same voltage for calculating the measurements. One phase, but two instances that are part of that same the device can measure, otherwise it will give you wrong readings.
The way you want it to measure on two phases, it will give you wrong reading on the second clamp. If possible, use two Shelly EM.
Hope this clarifies your inquiry.
Kind regards,
Shelly Support Team