Ok.....before it gets confusing.....if you are going to use seperate chambers for each driver, it is recommended to use equal volumes for each so the responses match in output and driver characteristics and control. As for using a common chamber, this can allow for a smaller volume requirement due to the drivers utilizing the same compression, so it may not be required to use exactly double the space for a common chamber. And as far as a chamber being too small....that is not a problem if the loading is controlled then by the port characteristics to obtain the desired output/tuning and as long as the drivers fit within that chamber properly. There is always an optimal volume and port for each sub. Its just a matter if it fits your personal preference or not. So, recommendation are merely a starting point in most cases.