Electronic – How to connect 2 different power supplies together

No, I would not tie the grounds together for such a distance. That does make the data connection more tricky though. In your case, the two grounds shouldn't be all that different since presumably they are tied together back at the breaker box in your house. In that case you could use differential signalling, knowing that the common mode offset should stay bounded to a few volts at most.

However, I think real isolation of the data signal is a better idea. If you can use ethernet, then you don't have to do anything further since that is already isolated. Otherwise, it's probably easier to use a opto-coupler at the receiver. The transmitter would send pulses of a few mA, which is enough to light the LED in the opto at the other end.

One simple scheme is to use UARTS, with the line idle level being no current. Cheap optos can be surprisingly slow, so either get fast optos or run the UARTs at a slow baud rate. Even a really slow opto can support 9600 baud well enough, and you can go much faster with the right choice of opto. If you need high data rate, then ethernet starts to sound good.

Maybe I misunderstood your configuration but if it's like the following I don't see the problem of measuring the individual currents.

Or is this what you want to do?

In this case there is no way to measure individual ground currents since there is only a combined current so it only leaves the high side shunt option.

Note that there is a solution for high side measuring with no need for supply like needed for INA138.
I refer to a solution like ZDS1009 which uses a current mirror.