If you are in a double wide slip with a floating dock, then you are tied on one side (let's say port) and your neighbor is tied on his starboard side. Tie the boats together! Run just one line from a cleat on your boat to a cleat on your neighbor's with just enough tension to keep you both off your respective docks. Ask first, of course. This was common practice in a marina I was in many years ago, and I have been doing it where I am presently for over a decade. It helps both boats and is pretty effective.
On the lines of what Faster recommended. I like Faster's idea of using a bungee, it may be the solution of what I was concerned about. Neighbors boat is a Nordic 44 and weighs probably 3 or 4 times what mine weighs. We get a raelly bad ground swell that at times can make its way into the harbor (waves can be breaking with 50 foot faces just outside the harbor). When that happens, boats are moving as well as the docks (since they are floating). If I tie to neighbors boat, that boat could be moving opposite direction as mine, and when a common line were to go tight, would rip the cleat off my boat (the Nordic 44 cleats are much stonger than mine). The bungee might work, but then again when things really start to move, best is to just be tied up somewhat close to the dock (with fenders) and ride it out that way. If I have too much slack in the dock lines, then the boat moves around too much, building a lot of momentum, before reaching the limit of the dock line. I do have line snubbers in all my dock lines to try to relieve some of the stress on lines and cleats.