To get shock, current should flow.
Current will take low resistance path, so when bird sat on the HT line, the current flows only through the conductor because of very low resistance comparative to Bird.
When you hang with single Hand: No return path through you and already conductor is low resistance path, so no shock to you.
When you hang with both the hands: No shock you will get, because the same conductor resistance is very low compared to you.
Human body resistance is 1 Kilo Ohm.
