for the pinouts of many Hats and pHats, including yours.
So none of the general purpose GPIOs (if that isn't too tautological) are used. i2c is a bus, so that can be shared. I'm not sure about i2s.
In general anything used as an output could be shared, as long as two peripherals want the same signals. Inputs cannot be shared unless they are designed that way.
If one pHat leaves all the connections for another unused, it should be possible to share the Pi just by paralleling the connections. BUT all Hats that meet the full specification use one non-sharable connection for the configuration EEPROM. So the Hat spec limits each Pi to one Hat (if you've ever tried juggling multiple human hats, yopu'll understand why that is a Good Thing). If only one of your Hats is a full implementation of the Hat spec, they might work together.