What an interesting element to consider, that Tom and Daisy are "closely linked" almost as an element of foreshadowing that Daisy would eventually refuse to leave Tom for Gatsby. Let's begin by exploring the nuances of their relationship as they first invite Nick Carraway inside.
First we'll deal with the casual glances, words, and even jokes shared between them. I mention this because couples who are, in fact, not closely linked wouldn't share these kinds of little marital nuances. Poor little Daisy has a bruised finger, you know. Tom is responsible, but the laughter here is far from accusatory. It's a joke made with the knowledge that Tom is in fact a strong and macho man, one that any flapper in the twenties would love to land:
"That's what I get for marrying a brute of a man, a great, big, hulking physical specimen of a--"
"I hate that word hulking," objected Tom crossly, "even in kidding."
"Hulking," insisted Daisy. (12)
Very typical joking banter and mock-annoyance that can be found in a couple who is, in fact, closely linked. There are also even more minute instances like Daisy noticing that "Tom's getting very profound" and proving that she knows him inside and out (13). Finally, there's the shared conquest of linking Jordan and Nick showing that Tom and Daisy are part of a couple who (at least sometimes) shares a mission, for after discussing their plans "Daisy and Tom looked at each other for a moment in silence" (19).
Another hidden piece of evidence that purely shows that Tom and Daisy are tightly linked can be found in the discussion of where they live. You see, Tom is actually from Chicago. Tom is living in East Egg precisely because that is where Daisy wants to be. This is incredibly significant and, quite frankly, shocking. On page 6, Nick even says the following:
[Tom's] family were enormously wealthy ... but now he'd left Chicago and come East in a fashion that rather took your breath away. (6)
It suggests that it wasn't traditional to move away from the family money. Further, Tom expresses his determination on the subject a bit later:
"Oh, I'll stay in the East, don't you worry," he said, glancing at Daisy and then back at me, as if he were alert for something more. "I'd be a ... fool to live anywhere else." (10)
So, there you go: Tom stakes his entire life on moving to the East, plans to stay there, and expects Daisy to do the same. Their glance shows that this is, in fact, the plan. The plan doesn't change by the end of the book.
Finally, it isn't long before Daisy gets upset about Tom's affair and voices her frustration towards her daughter, ... and herself. In reference to the birth, Daisy says, "I'm glad it's a girl. And I hope she'll be a fool--that's the best thing a girl can be in this world, a beautiful little fool" (17). That, my friend, is exactly what Daisy believes. Further, that is exactly what Daisy is. Daisy is a "beautiful little fool." Daisy does think that it's "the best thing a girl can be." Daisy will continue to remain so, ... Gatsby or no Gatsby.
So, although I'm embarrassed to admit that I've never quite considered it before, the evidence above proves that Daisy and Tom are tightly linked. Further, this significant link foreshadows that their relationship, however flawed, is forged and unbreakable. No, Daisy won't leave Tom for Gatsby. ... And now I have a new element of foreshadowing to teach in my classes.