Yes, a bivvy inside a hammock, you understood right, it adds an extra barrier for wind protection and also an extra layer to trap heat, I've seen but not tried yet a method of sticking a bag in the bottom of the bivvy to raise it off the sleeping bag to create a dead air space to trap more warm air.Sleeping Socks have to be dry, woolen, no cotton, and not to tight!
Did I understand right, that you used the standard British Army Goretex Bivvy Bag IN YOUR HAMMOCKS???
I asked my self, if it's possible to do that and to let the Tarp away. Did anyone do this???