They are two different places. Usually, people will arrive in Chengdu firstly, and then take a bus to vist Mt. Emei. You could stay one night on the mountain. On the next noon, please catch a bus to visit Leshan. In the afternoon, you could take a bus back to Chengdu.

Well, if you are not familiar with this region, it is suggested that you book a tour guide in advance online. I found an itinerary on this website, which could give you more references: https://www.travelchinaguide.com/package/chengdutour5.htm Hope this helps!