I have two berkley lightning rods. I am curious... COULD i cut off some of the handle, turning it into a split-grip, making it a lighter rod? Would that somehow dampen the rod at all? Has anyone ever done this?

I modified one of my gander mountain rods to a split grip rod.  It did not turn out to well (visually speaking).  The rod still functions fine but looks awful.  I could not get all of the cork off the blank.

I don't think it will hurt the rod as long as you are careful and don't knick or damage the blank.

I have seen some people do this that looked really nice.

I converted one of my old extreme rods into a split grip rod. It was very easy and it looks great. To do this I just took an x-acto knife and  slipped it between the two cork rings I wanted to start the split at. Take the x-acto the entire way around between the rings to make sure that the two rings get separated. Next do the same thing where you want the split grip to end. Then take the knife and whittle down the cork between the two cuts. Take off as much as you can with the knife until you get down to the glue/epoxy. When you have as much cork off as you can get, take some 100 grit sandpaper and sand the epoxy and the remaining cork off. If the sandpaper won't take off the epoxy you can try a file as long as you're careful not to file into the blank. Once you have most of the epoxy removed take some fine sandpaper to get the rest of it off. After that I took some rod varnish and varnished the split grip area.

Note: This worked very well and was easy to do on an extreme rod but I have no experience with lightning rods.