Well, there is such a tool for windows:
http://www.nmm-hd.org/newbbs/viewtopic.php?f=8&t=631
What is need to do is to create two named pipes out with one tool, such as ffmpeg/libav, then let x264-audio get those two named pipes in simultaneously, like this:
libav -i input.avs -f wav -vn \\.\pipe\1001 -f yuv4mpegpipe -an \\.\pipe\1002
x264 \\.\pipe\1001 --audiofile \\.\pipe\1002 -o output.mp4