moving markers or selectors independently with keyboard shortcuts

[GuentherRuehmann]

hello, I am new in Pro Tools and I just explore the functions in recording and editing.
In a tutorial I read about methods of setting markers - I am not sure about the correct word "marker", maybe it is "selector"?
I recorded an audiotrack and played it back by pressing 0 on numpad. While playing back i pressed Arrow-Down to mark a start position and after some beats I pressed Arrow-Up to mark the end position. By pressing 0 on numpad I could play back the song by start to end marker.
Now I wanted to move the start and end marker by pressing plus or minus on numpad. This works. Then I wanted to move the start and end marker separately by pressing option on keyboard with plus or minus on numpad.
Or I tried CMD on keyboard with plus oder minus on numpad. This did not work. I tried with Shift additional, but it did not work, too. It looks like the keyboard keys were ignored completely. PT moved always both start and end markers simultaniously to left or right.
Can anyone help me ?