If you have a Swiss Travel Pass you do not need a ticket from Lugano to Locarno, as you already have a ticket. Your pass is your ticket. Trains run from Lugano to Locarno every half hour.
The Bernina Express buss you need te reserve seats for. I assume you are also doing the Bernina Express train? You will find all that on www.rhb.ch
Note that the Bernina Express bus only exists as a convenient extension to the Bernina Express train. It is just a bus moving people. It is not a scenic travel experience.