Short answer: One is for TV set up and the other is for broadband setup!
A Sky engineer will be allocated for a TV appointment, and an Openreach engineer for a broadband appointment.
Your appointment will require one or more engineers who may or may not need access to your property as some of the installation work required can be done externally.
Before you let the engineer(s) in, it’s a good idea to check their ID badge to make sure they’re who they say they are.