You will need a
y-adapter which uses all three wires, so don't try it with a y-adapter for fans!
Also the so called aquabus ID of one pump must be changed because otherwise the aquaero is not able to differ between both pumps. To do this the pump must be connected to the USB. Run the aquasuite and go to the tab "pump and flow rate settings". In the lower right you can see a field for the aquabus address, change it from 10 to 11 and save the settings into the pump.