How can multiple web devs use git to work on one page simultaneously?

We're looking into using git for a new system we're designing.

We currently have a production machine and a dev machine.
We use svn to keep production updated with our dev commits.

The dev machine has a complete copy of our codebase for each developer (just 2 of us currently).

Here's the file structure:

html/
  dev_timmy/
    stuff/
      thing1.php
      thing2.php
  dev_johnny/
    stuff/
      thing1.php
      thing2.php

If we were both working on thing1.php, I would make changes to the file in my directory, and he'd make changes to it in his. To test our changes, we simply include our dev directory in the url.

http://ift.tt/1O5JTcZ
http://ift.tt/1POss35

I'm sure having copies of code isn't the best solution.

But how can we view our own changes on the internet?
How can we make dev_timmy-based urls resolve to the version of the page that Timmy is working on?