Actually you can use two blogs with one theme no problem if you are on Linux. Simply create a softlink to the theme you are wanting to share. This is actually a no brainer unless the specific theme has been modified to require something specific to the blog installation it is installed under.
I have done this successfully many times. I am referring to Themes only; and yes, the new blog should be intalled in a different folder.
Of course this implies that you should probably make the theme files read only unless the intention is to allow modifactions on one theme span to the other blog. On the other hand, it is often easier to maintain two seperate copies.
For an example of this in action see:
http://www.soundtrip.com/
http://hband.soundtrip.com/
The only file not shared is the header because I wanted to do something funky with the flash player. Otherwise the files are shared across both blogs, one theme, one theme installation, no problem, very convenient.
Trip