Windows
Make a new shortcut to the SL exe file.
Then in that shortcut go to properties.
Then in the properties find the Target box.
It will say something like “C:\Program Files\SecondLife\SecondLife.exe”
Just after the last ” in that box put this in. -multiple
So it reads like this “C:\Program Files\SecondLife\SecondLife.exe” -multiple
That lets you open as many as your computer can handle.
(You can only have one login at a time to an account. So you’ll need a second account for the second SL client you have open. Etc. Max is 5 per household.)
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Linux
Find the file gridargs.dat in the Second Life installation directory
Edit the file and add ‘-multiple’
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Mac OS X
Find the client in Finder (usually “Second Life.app”)
Right-click and select “Show Package Contents”
Open the Contents folder
Open the Resources folder
Open the “arguments.txt” file for editing
Add “-multiple” to the “arguments.txt” file and Save
(An alternative method for Mac (which may also work for Windows?) is to use different user accounts on your computer: this needs an initial set-up, but will then involve being able to run Second Life as multiples without any technical changes to Package Contents.)
Set up (repeat for further multiples if desired):
In your main user account, open System Preferences > Accounts and click the + symbol
Give the new account a name and short name
For speed, don’t use a password, and do not give the new account Administrator privileges
Under Login Options, check Enable Fast User Switching
Repeat the previous step In your main account if not already checked
Regular use:
Log in to your main SL account in your main computer user account (or in its own user account if you already gave it one)
Use User Switching (top right) to switch to your new user account
In Applications, launch Second Life and login to another of your SL accounts
(watch your computer slow right down!)
Use User Switching to switch quickly from one account to another, and back
The disadvantages of this second method is that you cannot have both SL windows on your screen simultaneously. The advantage is that you can run both instances of SL full screen
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info taken from the SecondLife Wiki