Filing a Counterclaim
If your spouse has already filed for divorce, you can't file a petition or complaint of your own. Instead, you must file a counterclaim, but this in essence acts as your own complaint. It bears the same case number as your spouse's filing, but it's a legal action in itself. If she decides she doesn't want the divorce and tries to dismiss the case, she can't – she can only dismiss her own pleadings, and your counterclaim will move ahead uncontested.
When There Are Two Petitions
If you try to file a divorce complaint -- rather than a counterclaim -- in the same county where your spouse has already done so, the court probably will not accept the second document. You might be able to file in a different county, but the court must usually dismiss your action when it learns of the other filing.
In some states, you and your spouse can file for divorce together, submitting a joint petition or complaint. This usually requires that your divorce be uncontested, and you may have to submit a signed marital settlement agreement to the court at the same time.