So I figured up average damage for a human character (so no increased damage die) with an 18 in their primary stat. I assumed a +7 attack bonus against an AC of 15 which seems to be pretty standard. That means an overall 65% chance to hit.
Average damage for
Dual wielder w/ 2 rapiers 2 attacks at 1d6+4/2 avg damage 4.7
Longsword & shield 1 attack at 1d8+4 avg damage 5.7
Greatsword 1d12+4 avg damage 7.1
Max damage on critical was included.
Dual wielding loses out 2 different ways. It has a smaller damage die, and because of division mechanics, the odds of rolling minimum damage are increased. The range of possible damage results is 2,3,3,4,4, or 5.
The one advantage that dual wielding has is that the odds of successfully hitting your opponent at least one time is 87.75%. But even with your higher accuracy, your average damage is still the lowest of the 3 options.