Great find! Love these old saws. For some reason when I see one at a yard sale I have to buy it if the price is right. Hence I have several out in my shop. Commonly know as a double buck, double crosscut or as BosnMate pointed out a misery whip. Although I have to say that from personal experience they are a lot more fun to use than a single buck saw. The most common sizes you will find are in the 5-6 foot range although they can be found ten feet or more long .
Here's a video of a double buck saw team in action. These folks are really slow, a good team would be through the log in about half the time.