The first point of your modified two-weapon combat style seems to rely on another rule of the undeclared house: you must declare all your attack targets in advance and throw all the throws attack at the same time, before knowing which one hits and how. a lot of damage they do.
This is not a normal part of the rules, although it may be a common shortcut to streamline things at your table. The rules as written allow you to launch an attack, to know if it has touched, then repair its damage and know the results, before you can perform other attacks of which you are able . You can even move between attacks, which is very useful when you kill your first target with one attack and you have to move to engage another.
So, your modified fighting style is not unbalanced in itself, but rather absurd when you play with the rules as they are written because you normally do not make multiple attack rolls at the same time time. You may be able to rephrase the sentence to better explain how this changes the normal rules, then a balance review might be possible:
- If you use your bonus action for a two-weapon combat, you can launch the attack for your second weapon at the same time as you launch an attack against the same target with your main weapon. If you do, you can apply a positive modifier of up to +5 to one of the attacking throws, and a negative modifier of the same magnitude at the other throw of the game. ;attack. You must choose the modifier to use before knowing if one or the other of the dice resulted in a hit or failure.
I suspect it's too loud, although I'm not an expert at balancing. This gives you two main benefits:
First, if you get a mediocre result and a very bad one, you can dump the bad by -5 (which does not cost you anything because it will miss anyway) and get a +5 to the best. This means that you will almost never feel the breath when you attack twice. It's definitely better than taking advantage over an attack, since you also get +5 at the best of both throws!
Secondly, it helps when you have a very good roll and a mediocre one. You can take a penalty on the right throw to turn the bad into something a little better. This can mean that you go from one shot and one miss to two shots. If you have deducted the exact AC from the enemy (which is quite easy to do after being attacked several times, and that many DM will tell you, especially when the AC comes from the other side. an obvious thing, like the mesh, for example). know exactly what is the size of a modifier that you can safely apply. So, I suspect that the potential disadvantage of missing out with both attacks when it penalizes your best result will be almost nonexistent in practice. The house rule should probably prohibit putting a modifier of -5 on a critical hit (since a natural hits always 20, regardless of the modifiers or AC).
Some changes could make it a bit more balanced. You can ask the player to use the ability to decide how to apply the modifiers before throwing (for example, "I'll take a -2 on my dagger for a +2 on my short sword"). Or you can limit how modifiers can be applied. Maybe the negative modifier can only be set on the hand-free attack roll? Or only to the highest of the two roles, to prevent the dumping of a -5 on a clear lack?