So you probably would do this anyways and have your own ideas, given Jacob and Esau are probably the second biggest pain in the a**. Regardless I felt like making a suggestion in case it inspires you, I have no modding experience.

Basically, being insta-killed when either one of the brother dies is the worst feeling in the world. SO, let's have that not be the case. Let's have it so that if one brother dies, the other may live one :)
This needs to be balanced however, otherwise you may as well just kill one brother super early in order to play the run as the other safe and sound. To do this, you'd have to mess around with the stats:
My suggestion is that if one brother dies, the other will have his stats halved AND the one that is alive will gain a certain percentage of the dead one's stats (based off the chapter in which you died). i.e. Esau dies → Jabob’s stats are halved → Jacob now adds a percentage of Esau’s stats at the time of death to his.
My suggestion for the percentage of stats that are transferred are:
Chapter 1: 0% taken from dead character - stats for alive character at 25% of when both characters were alive.
Chapter 2: 20% taken from dead character - stats for alive character at 35% of when both characters were alive.
Chapter 3: 40% taken from dead character - stats for alive character at 45% of when both characters were alive.
Chapter 4: 60% taken from dead character - stats for alive character at 55% of when both characters were alive.
Chapter 5 and beyond: 100% taken from dead character - stats for alive character at 75% of when both characters were alive.

Naturally this might need some balancing, I'm not entirely sure how this would feel in game, but I did try my best to make it fair. Does three things:
- Makes it so that the meta play isn't to just give all the good items to one character while the other has all the useless things. While still possible for confident players, others should opt for balance so that in the eventuality of death, you're character that's alive isn't neutered.
- Makes it so that surviving as long as possible with both characters can be rewarding for the player, while still ultimately a loss if death does occur.
- Makes it so that death isn't unnecessarily punishing, so the characters are fun and have more unique gameplay considerations.

Anyways that's my suggestion. Sorry for the long writing, it's a somewhat hard idea to explain. I hope you'll make a rework of any kind to Jacob and Esau tbh whether it's my idea or not, cause they're in desperate need of an injection of fun :)