How to share a current sense pin on a single GPIO
The IS pin provides a constant current proportional to the load current of the selected output, so if you connect two of them together you should be able to read the total current of both selected outputs.
Note this caution (emphasis added):

Due to the internal connection between IS pin and VS supply voltage,
it is not recommended to connect the IS pin to the sense current
output of other devices, if they are supplied by a different battery
feed.

Edit: Alternatively--



simulate this circuit &ndash; Schematic created using CircuitLab