Can I read back the contents of the two RDAC registers of AD5242 or AD5282 (dual I²C compatible digital potentiometer)?

Yes you can read back the RDAC register content (RDAC value of a specific channel). However, you can only read the RDAC channel that was selected during the previous WRITE mode. If the channel you want to read from is different from the channel previously written to, a dummy WRITE command is necessary to select the desired channel. Please see the datasheet for details.