Hi Pierre,
The module should remain isolated, and you can connect the common to the 24 V DC and measure your differential pairs.
If you need much higher isolation then you should consider the 9206, but for 24 V the 9205 will do the job. All voltages mus be within +- 10 V of the voltage on the common pin
(+- 10.6 Volts is the absolute limit when you reach the limit of the A to D converter).
Hope this helps.
Cheers
Stephen