To get two voice sessions going at once you'll need a renamed copy of SLVoice.exe, a second server-facing port range for it to use, and a second port for connecting to it with the viewer. If you use the same viewer I do, from what I can see they didn't make this easy at all... The name of SLVoice.exe is hard-coded in the viewer, and the port range is left to default with no way to specify them. Here's how you can pull it off though, if you're up for an adventure:
1. Run the first SecondLife.exe and get in voice. You're half way there!
2. Make a copy of SLVoice.exe and name it SLVoise.exe (it's important that the name is the same length)
3. Make a copy of SecondLife.exe and name it anything you want. The name of SLVoice.exe is hard-coded inside and we'll change it with a hex editor. I recommend HxD:
http://mh-nexus.de/en/hxd/4. Open your new copy of SecondLife.exe with the hex editor, and use search to find where it says SLVoice.exe. In the column on the right, select the 'c' and change it to an 's' so it says SLVoise.exe. Save your changes and close
5. Make a copy of vivoxsdk.dll and name it vivoksdk.dll (again, important that the name is the same length)
6. Open it with the hex editor and use search, except this time set the data type to "Integer Numbers" and search for 22860. If your dll is the same as mine, the first result is the one you're looking for -- where it says 4C 59, carefully change it to 7C 59.
7. Search again for an integer, this time 22892. Where it says 6C 59, carefully change it to 9C 59. Save changes and close
8. Open your copy of slvoice called SLVoise.exe with the hex editor and use search to find the text string: vivoxsdk.dll. Highlight the 'x' and change it to a 'k', making it vivoksdk.dll. Save changes and close
9. Run your copy of SecondLife.exe and log in. You won't be able to enable voice yet. Open Advanced->Debug Settings
10. Find the setting named CmdLineDisableVoice and set it FALSE
11. Find the setting named VoicePort and change it to 44130
12. Find the setting named VivoxDebugLevel. It probably says
change it to:
From: someone
-1 -i 127.0.0.1:44130
13. Open Preferences->Voice. Disable voice and click apply. Enable voice and click apply.
14. Enjoy success!
It worked for me, please don't be mad if it doesn't work for you xD