Rare? Think about my question! The answer is EVERY CEO became a CEO for the first time once! Most will have sat on the board of directors before getting the top job, some might be in a deputy role or a CFO role. But they will have never been in the CEO position until they get their first top job.
Giggs is in exactly that place, he's deputy to the man with the top job. So don't try and tell us that what United appear to be doing is any different to any other business. It might not be the usual way of appointing a manager but if they follow through on the idea of LVG effectively training Giggs it will certainly be an innovative one. And one that is used in private business all the time.