IMO the Burgess Bird book is pretty challenging for year 1. My oldest is in year 1 this year and I tried reading the Burgess Bird Book to all my kids and it tanked pretty well! We will try it again later.
I think for your literature readings you should try to keep in mind books that will appeal to both kids. For science/history readings, target your DS and let your preschooler tag along or not. My 2 year old often gets a table activity while we are doing read alouds. Today she mentioned something from a literature reading probably a month ago that she remembered! There are a lot of great picture books for science that both your kids would enjoy. We like Bumblebee at Apple Tree Lane and Miss Rumphius especially.