It is difficult to purchase a bus ticket on the spot. Usually, the bus leaves Chengdu for Jiuazhaigou around 08:00, so passengers are suggested to purchase the tickets one day in advance. So, it is suggested that you take a flight from Chengdu to arrive in Jiuzhaigou directly. I think you could stay in Jiuzhaigou, and then hire a cab driving you to visit Songpan in one day. Finally, you could take a bus or flight back to Chengdu.