If both people are alive when the soul forge happens, killing one kills the other. In the case of soul forging with an object, destroying the object kills the person.
In Damian's case, though, he soul forged himself to Ygerna when she was dead. Damian could have undid the soul forging after Ygerna was resurrected, or more likely (as apparently that made him stronger) killing her may have broken the soul forge or merely reverted it back to the original situation.
Also, if Damian died there would be no need for a Divinity 3.
If you release Behrlihn he shows up in the final battle with Ygerna, though not recovered from his imprisonment, so it doesn't make the fight very much tougher.
As for Ygerna's lack of a scar, have you seen Laiken? Damian has considerably more resources and more concern for appearance than Laiken did.
