-$cmd1 = "$sim1 $simargs --stats:file=tracediff-$$-1.stats 2>&1 |";
-$cmd2 = "$sim2 $simargs --stats:file=tracediff-$$-2.stats 2>&1 |";
+# Run individual invocations in separate dirs so output and intermediate
+# files (particularly config.py and config.ini) don't conflict.
+$dir1 = "tracediff-$$-1";
+$dir2 = "tracediff-$$-2";
+mkdir($dir1) or die "Can't create dir $dir1\n";
+mkdir($dir2) or die "Can't create dir $dir2\n";
+
+$cmd1 = "$sim1 $simargs -d $dir1 2>&1 |";
+$cmd2 = "$sim2 $simargs -d $dir2 2>&1 |";