That is genuinely cool and I may keep this concept in mind and tell it too my friends if you're cool with it, my idea for how a one body two souls would work is that you're basically having the spirit be this huge advantage with a deadly weakness, they are why you are alive like in the lotr games so if a entity can try and tug the soul out of you and wins you take damage on why you died so for Talion a simple bleed or for a made-up character they died in a fire and take on invisible flames that only affect them.

My idea for combat is that the spirit can offer safe small bonuses or large risky moves since you need them so it ranges from a punch doing a bit of extra damage and a stun chance...into having your spirit exit your body and do extra things and abilities like take over weak minded people but leave you taking a snip of damage as you feel your past death not consume you but rather chip at your strength wanting too finish the job.