(I mean “guideline”, not “rule”. There are no rules, only guidelines)
I’ve written many times that I do not find a second attack to model anything well in D&D. Especially two-weapon fighting.
First, let’s be clear that I’m (mostly) not including “shield” as a weapon here. D&D already has rules for shields, and I’m—currently—not interested in changing that.
There’s an argument to be made that two-weapon fighting was actually pretty rare in real life, so it is OK if the game doesn’t support it. A character can still carry two weapons attacking with either one each round. There are even some advantages to doing that if the weapons are different.
I believe, however, that there really is an advantage to fighting with two weapons, and I want my game to reflect that. I don’t, however, want two-weapon fighting to be so good that a player is foolish not to choose it. (And the benefits of a free hand should also be considered!)
So, this is the rule:
When fighting with two weapons, for each attack the player designates a primary weapon. The attack is resolved as if the character is only wielding the primary weapon but with a +1 “to hit”. Fighting with two weapons does not grant any additional attacks.
If asked to rationalize it, I will do it so thusly: I actually believe that a second weapon most often plays a defensive role. But…this tends to be less static blocks or parries so much as creating an opening for the primary weapon. Of course, D&D combat is abstract, so such rationalizations are always suspect.
Also, I’m not entirely convinced that the specific secondary weapon makes much difference. They each have advantages and disadvantages. This rule doesn’t penalize a player for choosing to use a longer (more expensive) second weapon, but neither does it penalize a player for choosing a shorter/cheaper second weapon.
We will also “reskin” the dagger as a buckler:
A buckler is the same price as a dagger. It is treated as a second weapon rather than as a shield, so it gives the above +1 “to hit”. The wielder also has the option of treating the buckler as a primary weapon (a buckler bash) doing d4 damage.