You might want to open two files in two separate windows to compare them. Also, if you want to edit two files simultaneously, you need to open them separately. That is why you can change Notepad ...
Open the Finder and browse to the folder you want. You can type Command-A (⌘A) to select all. If you want to select a specific range, but not everything in the folder, click one file, hold shift, and ...
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