When filing as Married Filing Separately, you must include your Spouse's SSN on your return.
If your Spouse is/has filed as Married Filing Separately, and you have filed as Single, your return will be rejected for your SSN being used on another return.
IF this occurs, you will be able to fix your return and re-file it.
IF you have filed first and then your Spouse files after you, their return will be rejected for the same reason and yours will be processed as a Single filer.
In that case, when your return is processed, you will then need to Amend your return to correct your filing status.
After your Amended return is Processed then your spouse will be able to file their return.
