Yes you can refinance just the second mortgage there are a couple of things that may come into play. Sometimes the lender that is in the first lien position will not allow you to just do the second,because they do not want to go down to the second positon,meaning if you default on your loan they would be second in line to get paid.
Also you need to know that quite often the interest rate on the second is higher then the first mortgage and therefore by combining the two you may be able to get a much lower rate.