unwrap as 2 halves and overlay them in the uv editor window you will need to scale 1/2 at a time because the back map will need to be inverted and then matched to the front 1/2 if it is symetrical

then in your image editor open the map, and the texture     scale the materials as needed

duplicate it and invert it with the merge line centered as close as possible to the center of the map

merge the material/ texture images and edit to keep close to the map with a small overlap

open it in blender

for smal tweaks use live unwrap to move both front&rear verts to fit the image

as long as you line everything up correctly it will be nearly seamless or you could always use projection paint and unwrap from view to paint directly to the main uvmap