Don't necessarily agree with this. If it's less than the excess then it's possibly legally up to each other to repair your own water damage, you can not be held liable as you are not negligable, unless you knew about the leak and did nothing?
Never really heard of anyone discussing insurance from a purists view point before, simply, if it's structural damage then it should fall under the block policy, forget any arguements about common areas etc.
And it would be a brave management company I personally think that would step in to a dispute between neighbouring apartments in a potential conflict like this, but this point I'm not sure about.
Maybe talk to the broker?
Hope I've read your query right, best of luck.