There are probably a few tests that will break if you try to run two or more test-webkitpy's concurrently on the same machine. There have been requests for this, so we should probably fix it ;) At the very least, we try to start up the web server, which we probably shouldn't do.
see also: https://bugs.webkit.org/show_bug.cgi?id=46322 https://bugs.webkit.org/show_bug.cgi?id=46320 which may be related.
Closing ... I believe this should be possible as of r76326.