It shouldn't make any difference as the tags are preserved during export either way (the cursor set before or after the tag at the split). Generally, the first tag in the segment (if present) is hidden if the "Hide boundary tags" option is selected. Place the cursor to the left of the tag displayed after merging so that it could return to the original position at the start of the second segment when you split it again.
Let's suppose I have accidentally merged two segments and would like to revert this action.
Where should I split to "undo" the merge action? At which point of the merge segment? Before or after the tag?
In my experience, when I try to split, there is a tag that is left, and some formatting can be messed up, hence my question/confusion.