At any time you may switch between forms gaining a +2 to str and -2 to int when in celestial or demon form and you return to normal when you are in your human form. However when you are in one form and drop to 0 you immediately revert to your other form with half of your maximum hit points (These points can be restored with a long rest) then roll 1d6 that is how many days until your other form regenerates but within this time if you are to drop to 0 you do not fall unconscious instead you are both separated and perish 

Even while you both are combined into one vessel you give off the aura of two beings. This means you are harder to read than the average person, when anyone who doesn’t have true sight makes an insight check against you they have disadvantage. However when people succeed they gain the knowledge that you are not alone in your body. 

While you share a body weapons are not shared between forms where as you must have a weapon for both forms because they switch with your transformation.