Shabbos 35 – שבת לה

The Gemara concludes that we are meant to conduct ourselves stringently regarding the two opinions of bein hashmashos (i.e. melacha on Motzei Shabbos can only commence after the conclusion of the later bein hashmashos).  Since bein hashmashos in itself is a period of uncertainty (safek day safek night) why is melacha not permitted during the later bein hasmashos  since it is a double safek (safek sfeika)  1) maybe this moment is already past the real bein hashmashos 2) maybe bein hashmashos in itself is really night; as we know a double safek in Torah is permitted?