Data sufficiency questions do not ask you to find the answer. They ask whether the given statements are enough to find it. That distinction changes everything: you stop calculating as soon as sufficiency is clear, and you never borrow information from one statement while testing the other alone.
Worked example: "What is x? Statement I: x + 7 = 15. Statement II: x is even." Statement I alone gives x = 8, so it is sufficient, and statement II is irrelevant. The answer is "Statement I alone is sufficient". Notice that no further work was needed — finding x was never the task; judging sufficiency was.