My understanding is that the reason for the single blank round is to give plausible deniability to the members of the firing squad (in that none can be sure that they actually shot a live round at the condemned person), while ensuring that the execution is swift and sure.
The opposite (only a single live round) makes no sense. People survive single gunshots all the time, and under the stress of the situation, it’s easy to miss the target completely. Look at the results of the Slovik execution. While he was hit by 11 bullets, only four were adjudged to be fatal.
(And BTW, Slovik was an idiot. He was so thoroughly convinced he’d only get jail time for his desertion that he refused multiple offers of clemency. He basically goaded his commanders and military judges into executing him.)