1. Download this script that comes with a test mission and launch the mission: http://www.armaholic.com/page.php?id=19048
2. Use the scroll wheel to select radio menu and join "Channel 1".
3. Use next/prev channel keys to select "Channel 1" chat.
4. Use the scroll wheel to select radio menu and join "Channel 2" (the scripts makes you leave "Channel 1" using radioChannelRemove).
5. Press the chat key, "Channel 1" chat is still selected as default chat.