I presume you are interpreting Sunder to mean that there is only one fracture? The item is "Split in Twain," as it were? A reasonable assumption, especially when considering that the spell talking about sundering an object as opposed to Shattering it.
But I think Shattering involves reducing the object to thousands of splinter-sized bits - Mending won't really be practacle. Sundering might be fixible with Mending, yes, but I think the object would still be reduced to multiple, large chunks and need several Mendings.
Of course, this particular nuance I don't think is specifically covered in any rules, so we're left making our own interpretations.