Have you different addresses for the two PCF8574s by changing A0,A1,A2?
First, thank for your answer.
Yes, I have.
One thing is 0x20 (all Ax connected with ground), the other is 0x21 ( only AO connected Vdd), But both didn't worked
If I remove one of them on the I2C bus, other worked ok.
Does the Edison detect both PCF8574?