Canon 85mm F1.8 have 9 elements in 7 groups, while 100mm F2 have 8 elements in 6 groups, and the diameter of the front element also differs. So are two different optical designs. I'm not in my hands 85 and 100 at the same time to make a direct comparison.
But I have Canon 50mm F1.4 and 100mm F2, and did side by side comparison.
The result is that the sharpness, contrast and longitudinal CA shown with Canon 100mm F2 wide open, is comparable to 50mm f1.4 when stopped down to F2.2.
Maybe I was drawn to the 100mm F2 best of all time.
For best performance, the only options for truly usable images in F2, are the 85L and 135L.