From Mac, I was able to open multiple instances by simply duplicating the Runescape legacy client and opening both. You must open them manually as Runemate prioritizes only one application and when attempting to create 2, the action will be denied. IF you want to open multiple instances of OSRS however, the contents of your application must be altered, as in your Info.plist. In order for your legacy client to launch an OSRS instance, open your Info.plist (I used TextMate) and around the 50th line (cmd+f to find the following code IF you marked "Use F1..." for your keyboards settings), change the "
http://www.runescape.com/k=3/l=$(Language:0)/jav_config.ws" to "
http://oldschool.runescape.com/jav_config.ws", save the file, and you should be able to launch OSRS with the legacy client. (Don't forget to do this for your duplicate also). IF you do not want to duplicate, then simply open your terminal and type in " open -n -a "Runescape copy.app" " (Don't include 1st and last quotation mark) and repeat for as many instances as you desire.
P.S This is assuming you had trouble starting a bot with the official OSRS client due to AgentLoadException.
Source:
(Terminal or Duplicate)
Configuring the client to launch OldSchool (Info.plist)