It's not clear what program you're working on, the ACOM on-line tree program or FTM?
Here is what you should be doing in the ACOM on-line member tree program.
-Add the spouse
-Create *add a fact* a marriage event in the timeline. If you don't know the exact date, ball park it. This links the couple. When you do find a document, just link it to the existing marriage event and edit in the correct date and place.
or.... you can start by creating the marriage event, the system will prompt you to choose or add a spouse.
This will sync correctly to FTM. Problem solved.