I've always used the cheat in Supta Kurmasana. Once in Kurmasana I would shuffle my feet towards eachother and work one over the other and tuck my head half under. Hadn't really bothered me too much that I couldn't get both legs behind my head and go down, not until I started moving into Intermediate and was facing up to Dwi pada sirasana.
At first I found Dwi Pada Sirasana funny, I would end up falling over rolling all over the place and giggling to myself time and time again. After awhile it stopped being funny and would end up in curses. Luckily Yoga nidrasana follows it which I find very relaxing, allowing me to get my yoga head back on.
Why couldn't I do it ? was it balance, shoulders too big, (especially after all the Karandavasana intensive), chest/back too broad......? But the weather has warmed up a little and I took my shirt off this week allowing me to use the sweat to slide my leg over more easily and Voila! OK, it's not pretty yet, not symmetrical, but it binds and stays put and I'm over the moon about it. the balance will improve now and I'll get used to bringing that left leg down a bit deeper.
Intermediate today so I can try Dwi pada sirasana and see if I can stay upright.