While you can install both (I've got SQL2012 Enterprise and SQL2008R2 Express on my work PC,) one of them will have to be a named instance.

IE if you install SQL2008R2 Developer first, it's going to grab the "default" instance, and SQL Express will have to be a named instance (which would default to "{Computer name}\Express"

If you install SQL2008Express, it will default to using a named instance, unless you explicitly change it during install.

The only way I can think of to achieve what you're looking for (both installed as a default instance, not a named instance) would be for one of the two to be loaded into a virtual machine (such as loading up Virtual PC, and loading a second copy of Win7 there)

You can check how your current system is loaded, one way is how you connect to each of your existing instances. My guess would be that the Developer edition is the "default instance," and the Express is a "named instance." In which case, load them up in any order, using the default options.

Jason