I've tried the following unsuccessfully to try and get multiple runs of XMLPatch to append to the log file:
Try 1
XMLPatch -o vanilla1.xml -d diff1.xml -u patched1.xml -l
XMLPatch -o patched1.xml -d diff2.xml -u patched1.xml -a
Try 2
XMLPatch -o vanilla1.xml -d diff1.xml -u patched1.xml -l
XMLPatch -o patched1.xml -d diff2.xml -u patched1.xml -l -a
Try 3
XMLPatch -o vanilla1.xml -d diff1.xml -u patched1.xml -l
XMLPatch -o patched1.xml -d diff2.xml -u patched1.xml -la