Plural or singular in "between the negotiated and fixed rate(s)"

It's acceptable as-is, I believe, because one rate is being compared to one other rate. (If a swap had multiple negotiated rates, or multiple fixed rates, then a plural 'rates' would be appropriate.) However, it is awkward because it does invite that sort of pluralization confusion. It would be better reworded to something like

Swap spread is the difference between a swap's negotiated rate and its fixed rate.

or, as jwpat7 suggests,

Swap spread is the difference between the negotiated rate and the fixed rate of a swap.

I agree with @jwpat7 said, that it would be most clear to repeat the word rate:

Swap spread is the difference between the negotiated rate and the fixed rate of a swap.

But to answer the original question, I believe that the plural rates would be correct.

Take a parallel example:

He is standing between the old person and young person.

Here you are talking about two people, and to use person just doesn't sound right.

He is standing between the old and young people. (correct)

He is standing between the old and young person. (strange)