A couple thoughts on your techniques:

1) If you want to sand before joining, consider aligning the leading edge instead of the trailing edge. Then, if there is overhang on one of the trailing edges, it can be sanded away instead of filling the leading edge. More specifically, I would recommend aligning the edges on the concave angle, which are the leading edge in your pictures.

2) If you join the pieces before rounding the edge, use an emery board to round the edge with the concave angle. You can get really nice boundaries where the two rounded edges meet. At least I get better results than I would get when just using sandpaper.