I carry all three, the thermo-lite bivvy,the heatsheat blanket and the heatsheet bivvy(during the colder seasons). I put one bivvy inside the other due to the velcro closing. I compressed the thermo-lite a little more and slid it down into a "ti" cup thus the cup takes up no space hardly at all since I carry the bivvy always. My Mil-spec glass signal mirror is stuck in between folds on the heatsheat blanket for prtection.
I have never had to use them(knock on wood) but lots of people say they are the way to go for good sleep/shelter when weight is an issue.