As for the rules of gaining an additional tomoe after the Sharingan's initial awakening, Sasuke clarified that it requires an emotionally distressing situation to gain another tomoe, and that it's something that can't be gained through training.
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Sasuke upgrading to his Phase II Sharingan seems to be an abnormality among the Uchiha clan, as his eyes aren't synced in their awakening, since initially he only gains Phase II Sharingan in one eye in the fight against Haku, simultaneously giving him a single Phase I Sharingan and a single Phase II Sharingan. I'm pretty sure this uneven gaining of tomoe is an almost unheard-of occurrence among the Uchiha, though we don't really see enough Uchiha gaining additional tomoe to really know for sure.
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It seems unusual that the Sharingan is even capable of unevenly upgrading in this way, though it should be noted that it's not long before Sasuke's other eye catches up and soon becomes a Phase II Sharingan as well, as seen in his later fight with Rock Lee. But evidently this proves it is not always the case that both eyes level up at the same time, though that is usually the case in most instances.
My guess for why Sasuke's Phase II upgrade happened so unevenly is because at the time of the awakening, Sasuke was somewhat trying to force his Sharingan to awaken, as he was trying to analyze the speed of Haku's needles with his regular eyesight, even though the speed of Haku's ice needles was impossible for the naked human eye to track. But Sasuke, knowing of his clan's superior vision, was probably somewhat trying to force his Sharingan awakening in order to save himself and Naruto.
After all, Sasuke's pitch black eyes start to lighten as he attempts to track Haku's movements, and with the strain of his effort he starts to be able to follow Haku's movements little by little. As a result, it's not a clean or immediate awakening, as his eyes don't flash instantly into Sharingan mode and immediately grant him access to the power. Rather, even before his Phase II Sharingan fully manifests, he is already starting to tap into the Sharingan's power.
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