I think the reason is the I2C interface. It seems that only one address is possible. 0x20. I will make some changes to the interface and see what happens.
What about your response in reply #2 "Yes, they have different addresses. I test this with a I2C scanner." ?
Have you dealt with any of the jumpers (solder blobs) on either of your Serial interface boards?
Have you considered the recommendations in reply #11?
Don