Automatically concatenating files using Watchr

I have a bunch of JS files that I split up, but want to concatenate automatically (so I don't define a bunch of files in HTML). Right now, I copied the command from Twitter Bootstrap. Makefile:

scripts:
 cat scripts/*.js > public/scripts/scripts.js

watchScripts: 
  watchr -e "watch('scripts/.*\.js') {system 'make scripts'}"

However, I've found watchr to be very inconsistent (on Mac OS X 10.8)

1. Sometimes when I save a js file, it doesn't run make scripts. Other times, it does.
2. One time, it just kept running make scripts continuously after a single save.
3. Sometimes, it will run the command a few seconds after the last save.

Am I doing something wrong? I'm working using node.js and not Ruby, so are there any node.js command-line alternatives? The same issues seem to happen to me when I try make watch in Bootstrap.

Aside question: I have another watch command:

stylus -w -u nib styles/styles.styl -o public/styles

How can I run two watch commands in a single Makefile command? IE make watch will watch both .styl files to compile, and .js files to concatenate. Right now I'm opening up two terminals, for each watch command, but I'd prefer a single one.