Just wondering how these two spells interact. If I was to cast debilitating portent on an enemy (who had failed the save) and he did 24 damage to me normally, it would then be 12 damage because of debilitating portent. With defending bone active would it be further reduced by 5 resulting in 7 damage?
Yes. Debilitating portent will lower the damage he deals. Defending bone will then reduce the damage you take.