Browserify
It is good at creating more than one bundle, but it cannot watch the directory and recreate a bundle. At least it does not appear to work.
Note
On Linux try running with sudo
to get watch functionality working.
Example building two bundles
GNUMakefile
browserify=node_modules/browserify/bin/cmd.js
uglifyjs=node_modules/uglifyjs/bin/uglifyjs
bextr=-r redux -r react -r react-dom -r react-redux -r redux-thunk -r react-google-charts
bextx=-x redux -x react -x react-dom -x react-redux -x redux-thunk -x react-google-charts
bundle-index: $(out) $(lib)
$(browserify) $(out)/*.js -o $(lib)/bundle-index.js $(bextx)
bundle-external: $(lib)
$(browserify) $(bextr) -o $(lib)/bundle-external.js
$(uglifyjs) $(lib)/bundle-external.js > $(lib)/bundle-external.min.js