I thought long, long ago in a world far, far away I had encountered some double-flat-blade screws. they were on some sort of older connector, iirc. apparently somebody else found them, for they got on Da Wiki...http://en.wikipedia.org/wiki/List_of_screw_drives
>> Cross-recess screw drive has two slots, oriented perpendicular to each other, in the fastener head; a slotted screwdriver is still used to drive just one of the slots. This type is usually found in cheaply made roofing bolts and the like, where a thread of 5 mm (0.20 in) or above has a large flattened pan head. The sole advantage is that they provide some measure of redundancy: should one slot be chewed up in service, the second may still be used.