One way to do this is to use the Crop Volume module to make the two new volumes of the two parts. Then you can assign a transform to one of the volumes and move it where you want. Then you can use Crop Volume again to make a volume that is the full extent
of the two volumes you want to put together and crop the rotated one into it. Then you can use Add Scalar Volume to add the other part of the volume. You will need to be careful at the seam between the volumes obviously and may need to mask one of the volumes.
Of course if you want to make all this precise you may want to write a script that determines the rotations and cut planes.