The question of how to run multiple instances of the Second Life® viewer appears many times in the SL forums. Here’s a way to do it that is apparently not well known.
With a current SL browser, you can set an option in the Advanced, Debug Settings dialog to allow you to run multiple avatars at the same time on one machine. The option is called “AllowMultipleViewers” . Set the value of “Allow Multiple Viewers” to TRUE. Once you set this value, exit Second Life. You should now be able to run Second Life with one account, and then start the program again and run a different account while the first instance of the program is still running.
This eliminates the need to mess around with your Second Life shortcut.