Double Ratchet Algorithm - What if the receiving wants to send the second message before the initiator?
I read again and again and I figure out how.

According to this section: 


  (This assumes Alice begins sending messages first, and Bob doesn't send messages until he has received one of Alice's messages. To allow Bob to send messages immediately after initialization Bob's sending chain key and Alice's receiving chain key could be initialized to a shared secret. For the sake of simplicity we won't consider this further.)


So, actually Bob's sending key chain and Alice's receiving key chain are initialized with a shared key. This allows Bob send the firsts messages of the Double Ratchet step (second+ messages).