In The Great Gatsby, what indication is there that Tom and Daisy are closely linked despite their marital difficulties?
In answer to your question concerning Tom and Daisy in The Great Gatsby, I suggest that there isn't much indication that Tom and Daisy are closely linked.
The only bit of evidence that they are, occurs when Tom mentions a few good times they once had and Daisy is convinced that she did at one time love Tom. She refuses to announce, as Gatsby wants her to, that she never loved Tom.
This is hardly conclusive evidence of how closely linked they are. All this amounts to is that Daisy wasn't pining for Gatsby the last five years like Gatsby was for her. That's the point in this scene.
Tom and Daisy have a terrible marriage. They do not respect each other, they do not communicate in any meaningful way, they both have affairs. In a time of stress they choose to maintain the status quo. And Daisy, showing her realism and character, refuses to pretend that Gatsby's illusion is true when it isn't.
This moment at the end of chapter 7 is one in which Fitzgerald only allows us (and Gatsby) to see gesture. I think this is purposeful. Had he chosen to let us in, there would be no room to let our romantic minds to wander to the degree at which the two of them reconcile.
We readers see them move together and they could be disappointed in their speech with one another, but all we see is the gesture of love. Marriage is tough. The situation they deal with (murder and adultery all in one day!) is one which requires some thought and for them to begin dealing with it show incredible stability. Couples with weak marriages couldn't very well consume the events of this day and be united. Somewhere in their past, they have been united and connected and this disarray has drawn them back together.
I think that you are probably talking about what happens between Tom and Daisy Buchanan at the end of Chapter 7. This is the chapter where she has admitted she loves Gatsby and where she has killed Myrtle Wilson (who is her husband's lover).
At the end of the chapter, the two of them are sitting together in a way that shows they are closely linked. They are sitting together and talking earnestly. His hand is on hers. She is looking up at him and nodding every now and then to show she agrees. So it is quite clear that they have at least papered over their issues and are once again close.
There was some indications in the book, but there were not many. One example would be when when Daisy tells Gatsby that she loved him but she loves Tom, too. She said that in chapter seven, when everyone had gone into town because of the hot weather outside. Another example would be when the book mentions Daisy's daughter, Pammy. Pammy is the daughter of both Daisy and Tom, and it's pretty self-explanatory as to what I mean when I talk about there daughter, I hope.