If you’re searching in Thunderbird, use the pipe (|) operator to do an ‘or’ search. So, if you want to find mail from both Brian and Grady, search for ‘brian|grady’ (if you have the default ‘search subject and/or sender’ search criteria). I looked around the Thunderbird Help, which has a helpful list of shortcuts and tips and tricks, and didn’t see this tip mentioned. I tried other special characters (& and *), but neither worked.
This was with Thunderbird 184.108.40.206.