It won't let you install Internet Explorer 9 32-bit only. It will then tell you that you need to install the 64-bit version, which will fully upgrade you to 9 beta. If you just want to try the engine of Internet Explorer 9 but still keep 8 around, then you can install Platform Preview 5 of IE 9, which is the engine that the beta uses. Just go to Internet Explorer 9 Test Drive
and click on "or get Platform Preview 5" and follow the download link.
Alternatively, you could set up a Virtual Machine to test Internet Explorer 9.