what would happen ?

Welcome to the forum SapcedInvader,

In general, the issues are... If you have a 24 VDC battery bank and draw 12 volts from 1/2 the battery bank. It will "discharge" the first time fine... But you will end up with the "high 12 volt" bank and the "low 12 volt" bank with different states of charge--And it will be difficult to get both battery sets back to full charge without damaging one of the 12 volt banks.

Not to say that this cannot be done. There are ballancers that will take power from the full charge bank and use it to charge the discharged bank. This is not uncommon in the RV industry where you may have a 24 volt chassis battery (run the coach chassis power) and 12 volts to run the "house" power (12 volt inverter, 12 volt lights, 12 volt appliances).

There are devices that can drop the 24 VDC DC power to 12 VDC (to run 12 volt appliances)... Just be aware that inverters typically draw a lot of current (i.e., 600 Watts @ 120 VAC or 5 amps; will draw >50 amps from the 12 VDC bus).

Cannot really give you specific answers without knowing more about your power system (24/12 VDC side, your batteries, 24 VDC inverter, your 12VDC/120VAC power needs).

-Bill