Don’t! Microsoft tries to warn you in the install.. if you are a cowboy and just want to get SharePoint Designer 2007 installed to accomplish a specific task requiring it, don’t install it on the same workstation as SP Designer 2010. Put it on another VM, do the task at home, wherever – anywhere but the same box as your dev SP Designer 2010 install.
Short of some registry hacking it looks like although SharePoint Designer 2010 will function in it’s base form after uninstalling 2007, if you try it open a site from the SP Site Actions menu it will fail to make the association and prompts you instead to “download SharePoint Designer”:
Full uninstall of both SP Designer 2007 and SP Designer 2010, resintall 2010. Same result: your SP Designer will have little birds circling it’s head and X’s for eyes.
I’m sure the answer is out there somewhere on how to restore the association between the site actions “Open in SharePoint Designer” link, and I will post here when I find it. In the meantime, all indications continue to it being wise not crossing the streams with 2007 & 2010 Designer!