The MultiPlus has a single AC input and two AC outputs. If you have two power sources such as generator and shore power you have to provide an external transfer switch to manually select which input source to use.
The Quattro had two AC inputs and two AC outputs. The Quattro will automatically select the active input and so you can eliminate the need for a transfer switch and make operation more automatic and user friendly.
The two outputs have the same function in both cases. One output only works when an external power source is available (shore or generator) and one is available to work at all times (inverter, shore or generator)