Thank you for posting this question to the Support Community.
You are correct that you cannot create overlapping schedules, but I believe you could implement what you described by setting the times for this special case in an unused system mode, like Custom1 mode.
For example you can set Custom1 mode to trigger Mon - Fri from 6AM - 8AM AND 5PM - 6PM. The rest of the system mode schedule would operate as it does now with Default, Lunch, and Night mode. Then for the extension that needs this special handling, use normal coverage with ring groups to match your desired call flow for Default, Lunch, and Custom1 mode. That should cover 6AM - 6PM on weekdays. The alternate call flow would need to be set for Night mode.
Alongside this, all extensions across the system that currently utilize coverage with system modes would have to be edited to mimic the correct coverage settings when the system is in Custom1 mode. If I'm not mistaken, all these other extensions would need to replicate the Night mode coverage settings during Custom1 mode.