You can stack them without the stagger but you must make sure that they are securely pinned together. Depending on how much 'push' is going on behind the retaining wall, you may need some buried anchors that run behind the wall.
3 to 4 foot pieces placed perpendicular/occasionally as "dead men" back into the wall can help to anchor a low vertical retaining wall. I made a 2 1/2 foot, pressure treated timber retaining wall 25 years aqo and it is still standing. I am amazed.