Would it be correct to write:
She were in bed when I arrived?
Your answer may depend on what sort of English you yourself speak.
Many of us do say she were, but it is not standard English.
Here is the sentence in standard English: She was in bed when I arrived.
When you write at work, use standard English.
One negative is all you need – Many of us use double negatives when we talk: He didn’t do nothing. Both didn’t and nothing are negatives. Used together, they become a double negative.
Logically, someone who didn’t do nothing must have done something.
We do not use double negatives (two negatives together) in writing.