Today, I am mostly trying to remember how to work a double start cast on without getting my tails tangled and, also, realising it is far harder to design a soap sock than I expected it to be.
Anyway, the double start cast on is a nice, quick cast on and has far more stretch than a regular double (long tail) cast on. As with the pesky long tail cast on, the super important bit is not getting your tails tangled and remembering to catch the correct strands. (If I'm talking double Dutch to you, there is a lovely video on Youtube that I watched recently, which demonstrates this cast on method.)
My long tail cast on mnemonic is "under, under, through" so my double start cast on mnemonic is "under, under, through; reverse; under, scoop, through". It's not catchy but it works for me. What works for you?