Who does on re-assign a detachment from one army to another?

[ulver1]

I realize I can merge them if they are in the same hex but suppose they are not?

[Tamas]

Not sure I understand your question, but you can re-assign a stack from one army to an other in the reorganization window.

[Jorrig]

Just drag and drop the detachment on one of the army listed in the box below that detachment. You may see an error message in the log when this action is not allowed, in case nothing happened. This happens mostly because of overstacking.