Merge not possible with big files

I tried to merge 2 big shapefiles, 220 Mb each. This causes an "out of memory" crash during the write step, after about 70 to 90 percent. I tried on 2 pc, with and without debug, machines have 8 and 16 Gbytes. Qgis/MMQgis requires about 30 mn and generates a 490 mb (all sizes include dbf, shp...)