Read current from two ground-connected supplies
If both 0V shunts are connected to power supply ground and then you connect the load end of the shunts together, electrically you get one shunt resistor and both readings will be the same.

If your "real" need is to have no shunts in the ground connections (so that both loads can be commoned to ground together) then maybe a high-side current measurement circuit is going to be better.

Here is a chip from TI that can do the job. It's a INA138-Q1