How the stats differences are is secondary, I used mine just as an example - but they should be opposite of the demon side, to balance them out. that's what I meant with bonus/malus = balance

Now, just imagine, I had a female mage and a demon warrior inside me. He would like to fight, using his strength - but she needs to defend/attack with intelligence (mana=spells) - then imagine, she would be in a magic restricted area and he keeps nagging at her that his sword needs a bloodbath. She knows, this is dangerous for her, can you imagine the dialogues? I guess, she would be very happy to have a succubus (female demon) in her. The int. lvl would be equal = double, but that would be no balance, merely owerpowering.

it's just my guess, inner conflicts (their dialogues) would strengthen the rift atmosphere - you have to balance over a rift, so not to topple over, this could be represented by the soulforged, too. And I believe, this would be a splendid possibility for the sadistic Larian humour I like so much.
Kiya