This is not what the function does. It is what it's being used for, but that's not the right kind of thing to document here. Something like this would be better:

/**
 * Ensure the FCurve with the 1st RNA path gets the 2nd RNA path, and vice versa.
 * \param list the list of FCurves to use to find the curves.
 * \param sock0_rna_path the first RNA path.
 * \param sock1_rna_path the second RNA path.
 */

Maybe you'd also want to change the code to count from 1 instead of from 0, so that it's more natural when the documentation mentions "first" and "second" (I'm never a fan of "zeroeth" and "first").

The name doesn't really convey the functionality of the function. It doesn't swap the fcurves, it swaps their rna_path. swap_fcurve_rna_path would be a better name.

I wouldn't mind if this function were to be moved to versioning_common.cc, as it's generic enough to be reused later.

Change ListBase *list to ListBase /*FCurve*/ *fcurves, so that it's clearer what's actually contained in the list (note that I added a comment and renamed the parameter).

You can return early here, and avoid an else. This also makes it clear that after this code, nothing more happens (instead of having to look down and inspect the rest of the code to come to the same conclusion).

It might be nice to have a little documentation here that says what the code does.

This should get a comment as to why the * 2 is there (and why it's enough).

Swap the condition & continue early. This reduces the cognitive complexity of the remainder of the loop body.

Keep formatting changes out of functional patches.