As a SharePoint administrator, You may have to have both SharePoint Designer 2007 and SharePoint Designer 2010 on the Same Machine to Manage SharePoint sites. Because SharePoint designer 2010 can’t be used for managing SharePoint 2007 sites, same way SharePoint Designer 2007 can not be used with SharePoint 2010 sites.
Technically, You can have both SharePoint designer 2007 and 2010 on same machine. Install SPD 2010 first and then install SPD 2007. This will work, however as per this Technet article its not supported!
The supported scenario is: Install SharePoint Designer 2010 32 bit version (Not only SPD 2010 but Office 2010 also must be x86 -32 bit version) and then Install SPD 2007 32 bit version (of course its available in only 32 bit). They must be in same bit-level version. So you can have SharePoint designer 2007 and 2010 on same computer!