The back on this bass had been shrunken and twisted so the centre seam split and no longer aligned.

Once the two sides were separated there was no way the edges would align without re-planing the edges, and this means adding 2mm of wood to the centre to restore the original width.

Jointing a back with a bend or curve in it, is tricky. You can't just line them up on a tabletop.

I built a pair of cradles to which I tack-glued the back pieces. This allowed me to align them on a flat surface, plane the edges and clamp the insert simply and accurately.

planing the edges flat:

glueing the centre strip:

... and before anyone says anything about the alignment of the upper bout in the next picture, this was just the two sides pushed up against each other before glueing!