I really dislike thinking about stories in terms of subplots. To me, subplots are either:
• An exploration of a character in detail
• A journey that has to happen in order to get over some hurdle
• A separate story
You see the first (an exploration) in a movie like Dead Poet's Society when one of the young boys goes out to a party and then drunkenly makes a move on someone's girlfriend. I don't think of it as a subplot because it's an essential demonstration of going after what your heart wants, which is essentially what the movie is about. This is the OS Concern.
You see the second in... well, the only story I can think of at the moment is an obscure book, but what happens is, these adventurers need to get their hands on a magical item, and they send a small group of people to get it. Meanwhile, the major plot continues to unfold. This is a Requirement.
Finding Nemo has the story of Nemo in the tank. It gets it's own storyform.
Anyway, your old couple could be a couple things. An exploration of the OS Concern maybe, or maybe they are the Consequence of the story -- they would have to Change their nature if they end up separated. At any rate, if you don't know where they belong, trust your storytelling gut to put them into the right place, and focus more on things like who the MC is.
If it makes you feel any better, the only way to learn the software is to plunge ahead and make mistakes. We all did it, now it's your turn.
PS. You can call me @mwollaeger because that will alert me to a message. Or Mike. Or всезнающий мозг if that makes you feel like you are talking to the right person.