Excel has a weird way of distinguishing between cells that are actually blank and cells that contain an empty string. Probably the only foolproof way to make a cell "actually blank" in Excel is to select the cell and press delete.
If your blank cells are actually blank, you can use this formula to merge AZ and BB (assuming you're on row 2):
If your blank cells contain empty strings, you can use this formula.
And merging the two formulas so it supports both kinds of blank cells:
You can also compare to 0 instead of using
ISBLANK, but I think
ISBLANK is more readable.
Edit: to address your clarifying comment: there's no way for a formula in Excel to create an "actually blank" cell. The best you can do is return an empty string. Since you want this to result when both AZ and BB are blank, you need to nest an additional IF statement in your formula.