I am assuming that we are talking one toilet? From what you say, sounds like this is a 57' with the convertible single/double beds at the stern, then bathroom then double bed.
In which case, there is ONE tank under the double, but it is spilt in two. The central section fills up first, thus not affecting the list of the boat, then it will start overflowing into the side tank, at which point the boat starts to lean over!
There are two emptying points, but it is one big tank. Timed pump out tend not to allow long enough to empty the whole thing out. Some places will try and charge you for two tanks.