Can we connect to an auxiliary relay switchover contact bothe 230Vac on NO and 24Vdc on NC without having interference on the Common Terminal of the Contact? Example of use: Switching between direct control of AC load and Input to PLC.

No you can't. As the common is the "same" point between the N/C and N/O contact, you have to switch the same voltage. Think of your light switch at home. The power must go through the switch for it to put the light on. So, you take the wire going to the light, break into it, and connect the wire at the bottom of the switch "common" and the other end of the wire at the top of the switch "N/O". The light needs 230VAC to work. If you activate the switch "N/O to N/C" the light goes on. If the light needed 24VDC to work, and you connected 230VAC on the common and you activate the switch, the light will blow as it needed 24VDC.

NO we can not it is out of regulation standard