Ex1. Hi, whoever is watching/reading this.
Ex2. Hi, whoever that is watching/reading this.
Are both correct? Or just 1? If so, why?
Thanks in advance.
IMO, only #1 is correct. "Whoever" means more or less "the person [or people] who/that is [or are] reading/watching this", so "that" in #2 is redundant.
Please be aware that I'm neither a native English speaker nor a teacher.