There will necessarily be some distortion when flattening, since although each piece is developable, the joined shape will not be developable across the seam (since the total surface angle around a point on the seam is less than 360°).
If the aim is to map a pattern from a joined flat shape to each half so that the pattern matches across the join, you could do this without actually modelling the 3d shape by making a surface from a copy of your shape but with the seam curve straightened, then using FlowAlongSurface to map a pattern from one to the other.
