been a while since statistics, but isn't the average roll of a d10 (theoretically) 5 or 6?
If that is true, then wouldn't the peaks be 5, 6, 15, and 16? (d2 being basically a flip of the coin)
If that is true (again, statistics was a while ago) this rolling method is much better for you, as you are more commonly rolling in a possible crit range, keen rapier being 15-20. The odds are different from the d20 roll, which peaks around 10 and 11.
Dafydd, you're confusing "average" with "most likely". With a single die, d10 or any other, every side is equally likely, so there wouldn't be peaks like that. Rolling two d10's to simulate a d20 would increase the average roll slightly because (mostly because) you can't get a 1; the lowest roll is now a 2.