Why can't I assign two substitutes to one job?

This an explanation of why you can't add two substitutes to a single day job with multiple segments.

Absence & Substitute is coded currently so that substitutes are assigned on a per-job basis and not a per-segment basis. The workaround for this is to create the single day job, with the multiple segments, and then split the job into two (or more) jobs. This allows you to assign the substitutes on a per-job basis. 

Learn how to split a job.