Yes, you can. However, before running each one, make sure that you run the thisfoci.bat script which is found in the Dev Studio bin directory for the version you want to run to flip the registry entries.
Finally I had a chance to verify your suggestion today. It did work. Thanks a lot. what supprise me is that after I run the version 7.6.6 for the first time, it even pick up the WebFOCUS Environments setting I had in version 5.3.3. I can still access the version 5.3 server from version 7 client.
Actually, I'm surprised at the environment stuff. The name of the file that stores the environment changed between 5.3 and 7.6. Go to Documents and Settings/yourid/Application Data/Information Builders. The 7.6 file is wfscom.xml. I don't remember the name of the 5.3 one but I know they were different.
And yes you will be able to access the 5.3 client from 7.6 but I would be cautious of that. If you try to create something using the 7.6 tools, it won't work in the other version.
I have three versions of Dev. Studio installed on my PC.
I find it helpful to create properly named desktop shortcuts to the three different focshell.exe and thisfoci.bat files. Then it's just two clicks on the desktop to change versions. No more trying to remember the name or location of the thisfoci file.