You can only have one true theme per site. It’s conceivable to have certain pages or posts appear to use a different theme by using different templates and CSS, but they remain part of the one theme. This sort of adaptation could take a lot of work, it’s almost like building a new theme.
It’d be easier to actually run two installations or a multisite installation. For two sites, I personally would still avoid multisite. More than two, then multisite starts to make sense.
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