Due to a limitation of USB and related audio drivers, there is no possible way to combine two or more AudioBox interfaces. You can only use one at a time. Your system may recognize that there are two plugged in, but the driver—the software that supports the system in using the device—is incapable of handling more than one of them, and may not be able to "decide" which one to talk to. This is not a limitation imposed by PreSonus, but a limitation in USB audio itself. FireWire and Thunderbolt interfaces can be daisy chained, and the drivers support aggregating I/O, but with USB this is not possible.