Sorry for the delay; I lost track of this one in my In box.
So if I have this right, you have a hyperlink in cell #1 that points to cell #2, and you want an easy way to create a similar hyperlink in cell #2 that points back to cell #1—and you don't need an automatic way to remove that second hyperlink later. Correct?
If so, I suppose one way to do that is to write an ordinary program that creates a hyperlink; the program can get all the information it needs if you make sure to point the cursor to cell #1 before invoking the program. The program looks at the current selection—that's cell #1. It looks inside that cell and sees the hyperlink pointing to cell #2. Now it knows what's needed, and it goes to cell #2 and creates there a hyperlink back to cell #1.
If you want to get a little fancier, I see there's a FollowHyperlink event for one worksheet or all of them. If you write a program to use this event, then when the operator clicks on a link the program will run; it can detect the cell that has the link and then create the new hyperlink as described above. Assuming you don't want all hyperlinks to generate new "back" links, you'd want your program to exercise a little discretion in its operation. How easy that would be depends on what criteria you'd want to use to make the determination.