Stacking is the option people forget and it is genuinely good for a specific case: a secondary screen you monitor rather than work in - chat, logs, a dashboard, documentation you glance at.
What it buys:
No horizontal turning at all. Your primary stays dead ahead. Glancing up is a much smaller movement than rotating, and eyes do more of it than the neck does.
Desk depth. Two side by side pushes you back or forces a shallow angle; stacked uses vertical space you already have.
What to watch:
The top screen must not be high. Looking up for extended periods is worse than looking slightly down. The top of the upper screen should still be around or below eye level when you sit up, which usually means the primary sits lower than people expect.
It is bad for a second screen you actually work in. Long stretches looking up will hurt.
It needs an arm or a stacking stand, since most monitor stands cannot do it.
So, for your split: main work left, chat and reference right - either primary centred with the secondary angled beside it, or primary centred with the secondary above. Both fix the rotation; pick on desk space and on how often you look across.