If I can elaborate on my initial response.
My understanding is that you have created a custom shared fact named Documented Spouse. When you attach this fact to an individual via the Person (Individual and shared facts) screen the fact is shared with the preferred spouse which may not be the spouse that you want.
What is actually happening is that the fact is being attached to the spouse shown in the mini-pedigree at the top of the screen. If you wish it to be another spouse then you have to change the spouse shown in that tree. Click the down arrow at the right end of the spouses name in the tree and select the spouse to whom you want to attach the shared fact.
Incidentally if you attempt to attach a shared fact to a person with no spouse then FTM will create an Unknown Spouse and the fact will be shared with unknown spouse.