chrome multiple instances

The way the portable app works, is another app (in this case GoogleChromePortable.exe or whatever the actual .exe is called) runs in the background that takes care of registry entries and such created by the actual app (chrome.exe I think it is). If you were to run the local app though, it would only run chrome.exe. Both chrome.exe files are essentially the same file, though, for both the portable and the static version.

If you were to run both together: a) you would possibly have conflicting registry entries being written/modified by both, causing potential issues with one version, or maybe even both versions, of the program, and b) depending on just things do interact, registry entries for the local version may be wiped by the portable version's handler exe, or registry entries and other data may be left behind by the portable version, which is a large part of portabilising an app is about.

Google Chrome is not the only app that does this. I know that Firefox is the same, you cannot run the portable while the local is running, and you cannot run the local while the portable is running.