I tried (without cheating) to complete both quests, but on giving the necklace back, Jonas is immediately killed and IIRC you can no longer attack Balin.
Maybe if you kill Balin by indirect means, then use the procedure in 'Reviving dead NPCs
'? Assuming indirect killing without Balin being hostile could complete the quest, you would have to restore a backup of the story.* files (I would guess) in order to undo the failed quest to return the necklace, so it could be completed.
Even if you could cheat to complete both quests, though, I strongly suspect you would still end up with one failed and one completed quest entry.