I love our Thirsties Duo Wraps. We used Bummis and Proraps covers for the NB stage. We probably started using the stage 1 Duo Wrap when he was about 10 lbs. I tried the Duo Diapers and I hated them. For some reason, they just fit J. differently than the Duo Wrap. They leaked every single time. So I sold them right away. I would have been able to double stuff them if I had needed to though.
I never tried to use our Thirsties overnight so I never tried to double stuff (since their regular inserts are awesome) but I'm sure you could if you wanted to. We didnt start CD'ing until DS was around 6 weeks old. At that point he was 14-15 pounds and the size 1s fit hum perfect when totally unsnapped. As such, I'm inclined to think they would fit an 8 - 10 pound baby well when snapped all the way down.
Re: Thirsties Duo-2 ?'s
I tried the Duo Diapers and I hated them. For some reason, they just fit J. differently than the Duo Wrap. They leaked every single time. So I sold them right away. I would have been able to double stuff them if I had needed to though.