I have tested those duel decks (because I test and analyse everything). And the Sorn deck definitely has the advantage.
The amount of lifegain in Sorin is not cool against a mostly red deck. The punisher spells are a joke, you will ALWAYS take the damage. The decks aren't adjusted to improve them, they're just as innefficient as in any other deck you'd put them in, except here your opponent can pretty much always afford to say no to them.
There' no contest here. Tibalt costs less, but he's also near useless. Where Sorin is all upsides Tibalt can, and often does actually hurt you. Throwing a land in your graveyard can hurt early, especially one of the few black mana prodcing ones. Plus, if your hand is empty, you get nothing out of his +1, while your opponent pops life-linking blockers.
Now, the Sorin deck doesn't have spells that outright kill Tibalt, but honestly, he starts low enough ,and you have enough creatures with flying that you can get rid of him pretty much anytime you want. And he's not that much of a threat anyway. Killing Sorin is a bit more complicated. If you don't draw flame javelin or devil's play. He'll probably stick around. Unlike Tibalt, Sorin makes his own blockers.
You have a single pyroclasm to try and survive the army of flying tokens coming out of efficient spells. And phantom general will make you cry. You have THREE spells that can kill it. And I don't know WHAT they were thinking putting vampire outcasts in there. 4 toughness plus lifelink. It can seal games by itself.
The sorin deck was given a silver bullet for some reason that I cannot find. decompose. The Card advantage you can get through the few flashback or unearth cards is the only thing giving you a chance as Tibalt. So losing even 2 of them makes a huge difference. Tibalt does NOT have a silver bullet card. The only thing that comes close is sulfuric vortex. Except it's double edged. And if Sorin gained some life before you played it (which it usually does) he's ahead and you're shooting yourself in the foot. Decompose has no downside. It's only a dead draw if Tibalt has nothing dangerous. In which case you don't mind. Vortex can MAKE YOU LOSE. That'S a pretty big downside.
Here's how I fixed the issue. Making Sorin weaker would make the deck less fun. And I felt it was more a matter of Tialt being poorly built. So I modified the deck to balance it against Sorin
1. gang of devils OUT. charmbreaker devils IN.
You really don't get enough out of that 6 mana with gang. Charmbreaker fits with the decks theme. And due to sulfuric vortex. I don't mind replacing a common by a rare.
2. 1 browbeat OUT , wild guess IN
Sorin really wants to deny you extra cards, and he gains enough life not to care. I left one in the deck, but wild guess is a better fit overall. It gets one card in your graveyard and gives you two. It's even a better deal than Tibalt.
3. Breaking point OUT flames of the firebrand IN
If you're at a point where you'd want breaking point to resolve, your opponent will take the damage unless they're at 6-7 life. The window of usefulness of this card is very narrow. You have to have gotten your (life gaining) opponent at a very low life total, but run out of threats, with breaking point probably the only card in your hand. Flames of the firebrand gives you a way to manage tokens, while still letting you remove things like phantom general or vampire nighthawk
Tibalt, the fiend-blooded OUT koth of the hammer or chandra ablaze IN.
The deck being dual colored keeps koth at an ok managable power level. Chandra the firebrand may be too good agaist Sorin. But then again the tokens have flying. Needs more testing, But there are options.
hellrider out, chandra's phoenix in.
Haven't drawn hellrider much. But phoenix has obvious synergy with the deck. Requires testing. If you have the decks try it and let me know.
This should make the games both more even, and make The tibalt deck more fun for the player.