I would not recommend mixing power supplies from two different panels, avoid at all costs. It may work, or you may end up having ground loops or other issues.
Also if you are bringing power in from a second panel, then it needs to be noted on the Panel that this disconnect does not remove all power sources, hence the Yellow Wire.
For the cost of a 24VDC relay, I see no point in taking that risk.
Depending on what the signal means, if it is critial, remember to make sure it is Fail Safe.
If you do not know what Fail Safe mean, you can google it, but basically, 1, or energized would be the Good or OK or Permissive State.
This way if a fuse is popped, or panel is powered down, or wire is cut, then the process waits or stops.
If you decide to send over Ethernet, then depending if the signal is critical, you would need a Comm Loss Alarm or something similar to detect communication loss.