it sounds like you're going to be working on different files anyway.
Notepad++ is free and alerts you whenever an opened file is modified under you.
Only the server code need be run from the same place for both of you. you can easily inject your own CSS and JS using userScripts and userStyles extensions for your browser.
That will let you mangle the code off-site.
in general, go ahead and make a backup folder and dump zipped folders there often, or use GIT to make revisions in case one day there is a conflict.
BROWSER STATS [% share]
(2013/12/12) IE7:0.3, IE8:7.5, IE11:4.3, IE9:4.6, IE10:7.8, FF:18, CH:43, SF:5.5, MOBILE:21.5