Can I run a 100 amp subpanel from a 100 amp panel?

You can run a 100 amp subpanel from a 100 amp main panel. A subpanel's rating is a maximum, not a demand, so a 100 amp box on a 60 or 100 amp feeder is fine.

What the code actually polices is the load calculation: everything on both panels together must fit the 100 amp service, per Article 220. The subpanel moves circuits around; it creates no new capacity.

Feeding it through a 60 amp breaker is common and often smarter, since the feeder wire gets cheaper and the main panel keeps headroom.

A 100 amp feeder from a 100 amp main is legal but tight; one big load in the sub can nuisance-trip the main.

The panel wire size calculator sizes the feeder for whatever breaker you land on.