The problem is not just that they're symbiotes but that they fill the same niche of challenging Peter's moral code. Venom does so by demonstrating what he considers a more effective form of justice and Carnage does so by showing that Peter can never stop him if he keeps doing things the way he usually does. Either of these on their own is fine, but together, they're worse than redundant; Carnage proves Venom right.
Because of these reasons, it can also be argued that neither of them work well for recurring antagonists. How many times could you stand to see them pop up and make Peter look like a weak fool before it just starts to feel like Batman and The Joker? Recurring Venom with no other symbiotes is just a tired song and dance of Eddie calling Peter a bitch and trying to kill his loved ones. Having him be recurring while also being essentially a razor-toothed Punisher also makes Venom's methods look ineffectual by showing that he fails to make any dent in crime because he either doesn't kill enough or more criminals pop out of thin air faster than he can slaughter them. If that sounds bad, just try to imagine Carnage as the only symbiote full stop.