Yes. Not being able to reach the swordsman.
Any semi-competent swordsman will be able to keep their blade between themselves and their opponent. At that point, they should be able to skewer the knife fighter without reprisal.
Actual combat isn’t like, say, D&D. It’s not that one character can deal 1d8 damage, and the other can deal 1d4 in two separate attacks and they’re standing in adjacent spaces. Your knife fighter needs to be closer to their opponent than the sword allows. If they try to close that distance, they’re very likely to end up run through in the process.
Real world combat involves a concept called “reach”. You see this one relied on a lot by some writers, but very few actually grasp what it means. Reach is the distance you have between yourself and your opponent, basically how far it takes to hit them. Hand to hand uses reach to define the different zones around the body, how far it takes to hit someone with your legs, how far it takes to hit an opponent with your hands, when you’re in the grappling zone, to define which techniques get used when. One usually transitions as you get closer, so from legs, to hands, to grappling, until we fall into ground fighting.
That’s very basic and the rules change drastically depending on the styles involved (more so than the body type). With weapons, reach or the distance it takes to strike your opponent is also very important. Different weapons come in different lengths, shortening or lengthening the distance.
Knives, particularly short knives, are most useful when supplementing hand to hand combat rather than weapon combat. They provide an added benefit when placed against the distance involved and blades are more effective than hands/fists.
When placed against a weapon with greater length, as I said above, like a sword it has a disadvantage because it must close a greater distance in order to reach the opponent. The same is true for the sword versus a longarm/polearm such as a staff or a spear. The sword must close a greater distance in order to strike, while the spear has the ability to strike from a safe distance where the sword cannot reach them. The situation can change based on environment and the people involved are very important. However, in the flat comparison of two weapons: the sword has the advantage because of it’s greater reach.
More importantly, the concept of the slow and lumbering longsword is a myth. Swords are very quick. The best advantage a knife wielder has in the scenario is to kill the swordsman before they can draw or be in an advantageous range before the fight begins. They can also work them into a position where the sword loses it’s advantage due to the environment or situation.
In traditional combat, the second weapon often serves a defensive purpose rather than a primarily offensive one. It behaves like a shield, locking up the other weapon and creating openings in the opponent’s defense so the wielder can strike. This won’t help two knives much against a sword, because reach is still an issue and the second weapon doesn’t negate it. Two knives don’t function in the same manner as say a rapier and a parrying dagger, but it is worth keeping in mind for when you’re writing your scene.