This is actually a common sense argument, but there is a specific issue here with regard to property insurance that you may not be aware of.
First, commercial property owners have a strong incentive to get as little insurance on their properties as possible, because any money saved goes directly to the bottom line (i.e., profit). IIRC Silverstein tried to get $1.5 billion in coverage, but his lenders insisted on $2.3 billion (just talking about the leasehold properties here, not WTC 7).
It is actually common for commercial lenders to require the full amount of their loan in coverage on the property. However, they may be convinced to reduce that requirement based on specific facts related to the property. For example, many lenders will allow a borrower to insure a multi-building property for less than the full amount of the loan per occurence on the common sense notion that even if one building catches fire, that does not mean that the others will, at least not as part of the same occurence.
That seems to me to be the deal Silverstein and his lenders came to; that the buildings could be insured for substantially less than it would cost to rebuild them, on the basis that even if one buiding were heavily damaged it was unlikely that the other would in the same occurence. Whether the insurers underwrote the risk that way, is, of course, another matter.
Now do you see why Silverstein has at least an arguable claim that the terrorist attacks were two separate occurences? I tend to agree that it's a bit tenuous, but it's not as unreasonable as it might sound at first.