For simplicity's sake, I'd say stick with one with two "site" subdirectories. That might be a better option. BUT, if you don't set XAMPP to auto-start, I don't see why you cannot install them to separate directories so long as you have the disk space.

Whatever floats your boat.

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Programmers are in a race with the Universe to create bigger and better idiot-proof programs, while the Universe is trying to create bigger and better idiots. So far the Universe is winning. - Rich Cook