I'd like to install Adobe Reader X but for various reasons I'm not quite ready to get rid of Reader 9 yet. Is it possible to install Reader X but avoid the automatic removal of Reader 9 in the process?
Some users find it helpful to mix and match the features and workflows from different versions of Adobe Acrobat to achieve familiar results with their documents. However, as a best practice, only one version of Adobe Acrobat should be installed on a single computer. The presence of two or more versions of the same program on one machine consistently leads to disabled functionality, printer conflicts, and PDF display errors in web browsers, among other problems. Nevertheless, for users willing to accept the risks, installation of multiple Acrobat versions is relatively simple.
Read more about it on: How to Install Multiple Instances of Adobe Acrobat