Theoretically, 127 devices can operate on a single USB interface, although in practice, they would require far more bandwidth in total than available. This means the communication has to be a lot more advanced than sending a constant stream of instructions back and forth. You don't want mouse movements picked up by a storage device and interpreted as a write signal, for instance.
Whenever a device is connected, the host sends a SETUP signal, followed by a 11-bit address, four of which are reserved for defining the device function, hence the limit of 127 devices. All subsequent requests to transmit or start receiving data, will mention this address. That's why simply splitting the cable won't work. In fact, it is often split internally already. Both printers will receive the instructions, but only one of them will listen.
Tricking the printer into listening to the address assigned to its neighbour, won't work either. Both slaves would reply simultaneously and the resulting signal will be ambiguous and/or gibberish. Therefore, no simple logical splitters exist that can link two printers the way you want. You'll need to install software that duplicates the print job before it gets sent to the USB controller.