To record that 2 people are a couple (married or not) you add one as the spouse of the other (in the Focus Window or using any other technique). When you have done this, you can record further details about their marriage using the Property Box (see How to View an Individual's Record in the Property Box). It doesn't matter which spouse you use when you do this. After you have entered all such details, You will find that all the marriage details will be equally visible (and can be changed or deleted equally well) when you are viewing either spouse's record details. This is because marriage details are actually stored in a separate family record, access to which is shared between both spouses. This is all taken care of for you 'behind the scenes' by Family Historian.
You can enter basic details in the Main tab of the Property Box. To record more details about the events and attributes that relate to their marriage (e.g. to record a note about the marriage event, or to record the event of their becoming engaged, or divorced, or whatever) you must go to the Facts tab of the Property Box. To add marriage events and attributes, click on the button and select all the events or attributes you wish to add. When you choose an event or attribute which applies to a couple, you will be required to confirm which couple they apply to. Family events and attributes (the ones that apply to a couple) have a red dot in the listing. Individual events and attributes have a blue dot.
See also How to Create a Note About a Marriage.