I will advise you not to do that. It is best that the Multi is able to carry the full load, because the Fronius unit may disconnect for whatever reason. For example, I’ve seen the Fronius disconnect at the moment of a grid failure (because it sees a drop in the voltage), and then it needs a little time (about 60 seconds usually) to reconnect. During that time the Multi must be able to carry the full load.
Also imagine this scenario: The batteries are full, so the Multi pushes up the frequency to around 52Hz to throttle the Fronius down. Then a large load (let’s say 8kW) starts up. While the Multi pulls the frequency down (which it cannot do too quickly, otherwise the Fronius will see it as an islanding event), the Multi has to be able to carry the full load.
So no, don’t count on this actually working. Rather consider at what time the power is more likely to be used (during day time as it is generated, or at night). If you use most of the power immediately, go with the Fronius. If most of it is stored for later use, go with the MPPT.