と同時に means "at the same time as". After an event, the second thing happens the moment the first does; after a description of someone or something, it says they are two things at once. A verb is in the dictionary form, and a noun naming an event takes と同時に directly.
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The moment the door opened, the cat dashed out.
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The instant the bell rang, the students left the classroom.
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I started work as soon as I graduated.
Two things at once
A noun or a な-adjective takes であると同時に, and an い-adjective takes と同時に directly. The second half often carries : 〜でもある, "is also".
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He is a company president, and at the same time a father.
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This work is art, and at the same time a critique of society.
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Graduating makes me happy, and at the same time a little sad.
と同時に or とたんに
also means "the moment", but it follows the た-form and carries surprise: what came next was unexpected, not something anyone set out to do. と同時に says only that the timing matches, and the second half can be something someone decided to do.
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The moment I stood up, I felt dizzy.
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The players started running the moment the match began.
is different again: one person doing two things over the same stretch of time. と同時に picks out a moment, and the two events can have different subjects.
と同時に sits comfortably in writing, news and announcements, and is fine in speech. For a casual "as soon as", say .
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Call me as soon as you get there.
Lingaku