Marriages in Schnapsen/Sixty-six
I received an answer on /r/cardgames. It's from David Parlett's Oxford A to Z of Card Games.

I recommend David Parlett's Oxford A to Z of Card Games. Here's my understanding from his book:
You lead one member of the happy couple, reveal the other (but hold onto it), and play the trick as normal. In your example, you would take the Jack, and also score for the King or the Queen you chose to play on top of the marriage points.

The key I was missing was the &quot;hold on to it&quot; part, even after the marriage happens and points are awarded (assuming you took a trick). This takes care of the problem I was having where my hand would have one less card than my opponent's.