As mentioned in the PCAL6408A datasheet:
There are two supply voltages for PCAL6408A: VDD(I2C-bus) and VDD(P).
VDD(I2C-bus) provides the supply voltage for the interface at the master side (for example, a microcontroller) and the VDD(P) provides the supply for core circuits and Port P.
The bidirectional voltage level translation in the PCAL6408A is provided through VDD(I2C-bus).
VDD(I2C-bus) should be connected to the VDD of the external SCL/SDA lines.
This indicates the VDD level of the I2C-bus to the PCAL6408A, while the voltage level on Port P of the PCAL6408A is determined by the VDD(P).
In Our design, VDD(I2C-bus) and the high-level voltage of Port P are equal (both 3.3V), can VDD(I2C-bus) and VDD(P) can be connected together?