In ADBMS6830 during OW test why two neighborhood switches should not be activated simultaneously. How it will mask the break of the common cable between these two cells

The Yellow marking represent the case where neighborhood OW channels are closed, Blue marking shows flow of current when only OW switch on the wire open channel is closed. 

If neighboring channels are closed, there is a path for current to flow from C2+ve to C1-ve through the OW switches OWSe and OWSo, thus the 100nF capacitor on S2 pin will not discharge. In this case, if the RED marked wire is disconnected (common wire between Cell1 and Cell2) we will not be able to detect an OW case as there is not current flowing in/out of that wire.

In case we only activate the OW switch on that channel the 100nF on S1 will discharge to through OWSo of S1 indicating us that there is a open S1 wire (or open S2-ve wire). This is shown in Blue.