Magnum inverter, Yamaha Generator

Their design (inverter gen) is totally different from generic or traditional generators. While generics make power directly from the magnet coils on both stator and rotor (some with permanent magnets) are delivered unmodified to the connected load; on the other hand, inverter type generators make power by making 12 volt first and then transformed to 120 volts.

You might not know the inverter generators as well as you think. I can't speak for every inverter gen on the planet, but of the major brands (Honda, Yamaha, Kipor, etc.), the only place anything close to 12 volts exists is in the lame direct battery charging feature. The output of the main generator is higher voltage AC, rectified to produce approximately 170 volts DC, which is the input to the inverter. Note that 170 volts DC is the peak voltage of a 120 volt RMS sine wave.

This scheme makes it easier to couple two generators by controlling the low side (12 volts) before it gets transformed into 120 volts.
OP is trying to parallel two power sources namely: Magnum Inverter and Generator. This an absolute no no.

With the Yamaha and Honda inverter gensets there is no mutual coordination of the pre-inverter DC bus. The only information one of them has about the other power source is its AC output, and doesn't have a clue what is producing it (shore power, another generator, or inverter). Earlier Kipors also worked this way until Honda forced them into a sync-cable design, also used by Champion, over patent violation claims.