I have no idea how to tackle that the good way 
but I have a bad idea
I would consider this new “fillet” curve to be created by point interpolation:
the X and Y coordinates of these points are controlled from top view (projection of the original curves on XY → fillet → divide fillet into frames → just take their XY coordinate, save the frames for later)
and for their Z coordinate?
maybe we could steal the Z coordinate by intersecting the frames with the “average blend” between those curves? (this is basically wrong from several points of view)
and here is a bad Frankenstein-like creature
[amount of control points depends on the fillet radius amplitude]
Frankenstein_of_bad_Stuff.gh (18.4 KB)
[edit] or we could just use curve from 2 views man_facepalming: 
fennec plugin has a pretty simple and useful component for that
Frankenstein_of_bad_Stuff_Fennec.gh (23.2 KB)