A somewhat dirty workaround, but since the projection on both files is identical, you can actually skip using proj at all by removing/renaming the *.prj files. Then merging should work.
- Bug#838915: saga: Segmentation fault in CSG_Projecti... Nelson A. de Oliveira
- Bug#838915: saga: Segmentation fault in CSG_Pro... Johan Van de Wauw
- Bug#838915: saga: Segmentation fault in CSG... Johan Van de Wauw
- Bug#838915: saga: Segmentation fault in... Nelson A. de Oliveira
- Bug#838915: saga: Segmentation faul... Nelson A. de Oliveira