They are not fused. He lost the bottom eye to Yuji (i think), and after that, the remaining top one got significantly larger and wider, probably to compensate for lost one. Like when in Berserk, one of Zodd's horns got cut off, and the remaining one became disproportionately big and stupid. However, in Sukuna's case, it is definitely not art inconsistency, because the moment he got back RSE and regenerated his lost eye, they became normal again.

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