There are no specific rules for firing a multiple blast, ordnance barrage weapon. For this reason, I think most people are happy to consider the Manticore as firing as if it were a unit of multiple barrage weapons.
Of course, if it's firing directly, then you resolve each scatter separately since it's no longer a barrage weapon.
Seems like the easiest way, for me, to resolve the lack of hard rules on the issue.
DoW