In general, investors would prefer two companies because those looking to invest in retail wouldn’t have to expose themselves to cloud & vice versa. That increase in demand for each component (investors who formerly ignored the stock owing to the above) would make the share price go up, all else constant.

Amazon stock performs well because a lot of investors want access to both, so it’s not that big of a deal. Also Jeff Bezos holds a disproportionate amount of voting power (part of why he’s the richest person is because he held on to an unusually high percent of an unusually valuable company).

Ultimately they won’t split until one of them stops being a world-eating monster because no one will have leverage to force it on Jeff. It may never happen as long as amazon exists. But I guarantee the day amazon cloud stops growing at insane rates, people will call for a breakup.