You can add another aquaero 6 LT as a slave. Since aquasuite 2017 you are able to use the four headers with PWM.
For the aquaero 5/6 LT devices there is now also the option to use the PWM outputs when the device has been programmed as a slave device. In this configuration, a master aquaero is extended by 4 additional fan channels by a slave aquaero. So far there was only the possibility to use the aquaero without PWM support.