I have try it, one PC was connected via ethernet and other was in RS232C connected to RSLogix5000.
When one or the other makes changes to PLC program that other can see it with small icon on the right bottom of the display and RSLogix tells program has changed, please save it.
Variable tables could be done misstakes if one or the other changes or makes newones.
If both looks same Rung, both can see if other are doing changes.