Can you get the other screw out w/o problems? If so, you should be able to sneak something up in the second hole and wedge it against the spinning retainer (small screwdriver, etc.) to create a wedge to hold the nut while you loosen the screw.
Seems odd the barrel of the retaining nut split from the base...
To help you "see" the design of the piece:
This is what mine looked like peering up thru the hole (see all the vert cracks):
if you were to remove the windsheild, top down view:
underside view if header was transparent.
Excuse the crudeness but a side profile view would be this:
__TT__
Try and get the second one out and see if you can wedge something up there to keep the barrel from spinning. Shouldn't be too hard as I recall as it's a well machined thread.
Good luck
