Once I launch the portable DO from the taskbar button, it launches a separate session, meaning I cud launch a separate session of DO again from the taskbar button.
However, once I launch the installed desktop version from the taskbar button, the taskbar button becomes the active session, i.e. there is no separate button anymore from which I might launch a 2nd session.
I then have to either jump to desktop (closing all active windows) or go via Start.
However it is just handy to be able to launch a 2nd session from the taskbar, same as with the portable version.
Taskbar button: the shortcut right from the Start button
Separate session: launch Directory Opus a 2nd time, i.e. I then have 2 sessions/open windows with Directory
Opus. Occassionally this is handy, e.g. when you wish to move/copy files to a location, but you wish to check out that location first.
You're getting confused by how the Windows 7 taskbar works.
If you have installed Opus on a machine and pinned it to the Windows 7 taskbar, and Opus isn't already running, then that taskbar icon will launch the installed copy of Opus (i.e. the taskbar will launch the program you pinned to the taskbar).
If you run Opus when another copy of Opus is already running, the new copy will just tell the old one to open a window, and then the new copy will exit.
Remember also that (by default) Opus stays running in the background even when you close all of its windows.
If Opus is running and has a window open, then the pinned button for Opus on the taskbar will just open that window. If you want to open another window, either shift-left-click or middle-click the taskbar button. That isn't an Opus thing; it's how the Windows 7 taskbar works with all programs.