How about creating a voltage divider and connect the single GPIO pin to the ADC and sample the voltage.
If both are at the high impedance state voltage will be VCC
if CHRG gets low, the voltage will be pulled to 66%VCC
if STDBY gets low the voltage will be pulled to 50% VCC
In case it's needed, if both get low it will be pulled further down, below 50% of VCC.