Parents and spouses have been added individually. How do I go back and add a marriage event between two people?
Double click either spouse to open their Edit Person screen. Initiate the adding of a new fact just like you were adding a Birth or Death fact except choose the Marriage fact. You usually initiate the process by clicking on the + icon to add a fact.
The dialog for the Marriage fact or any other couple fact such as Divorce will include an extra step where it asks you whether you wish to add the fact to this person and an existing spouse or if you want to add a new spouse. Normally, you would choose an existing spouse, namely the spouse you have already added. You enter a Marriage Date and a Marriage Place, and if you wish you also add a Marriage Note and a Marriage Source, just like you do with any other fact.
If you close out that person’s Edit Person screen and open the Edit Person screen for the other spouse, you will see that the Marriage fact is already there. Any future change you make to the Marriage fact from the Edit Person screen of either spouse will show up immediately on the Edit Person screen for the other spouse.
The way couple facts like Marriage work in RM is not the same thing as “shared facts”, which are something which is entirely different.