You have too many characters on a team to fully use FTL. FTL runs out of combinations after 26. The easiest way around the message will be to select your "second pc" Character Set in the top left pane, then in the bottom left pane select each Slot number (1,2,3, etc) and on the right un-check the boxes for Slot ID ("FTL") Modifiers -- all of them in this Character Set, because they ALL conflict with the ones in "main pc", because the combinations ran out on Character #27.
Whenever I multibox I keep getting short game freezes that make it impossible to play
You specifically mention orgrimmar and stormwind but I'm not really clear when else this occurs. It sounds like it has nothing to do with orgrimmar or stormwind if the game is impossible to play.
Have you ... tried .. anything? Over the years we have condensed the first stuff to try into
http://isboxer.com/wiki/HOWTO:Tweak_your_framerateI would change your CPU strategy though. Instead of Round-robin try selecting all instances use all cores. You can quickly apply this by using the CPU Strategy Wizard.
Beyond that the most helpful information you can provide relating to this issue is probably going to be "Diagnostics for game crashes" from ISBoxer's Help menu. This will list running programs and modules loaded into the game that might be having a negative effect.
Also from the in-game ISBoxer Control Panel (Ctrl+Shift+Alt+G) you can click the debug tab, and click the "Capture State" button. This will grab some info from all of your game windows such as their FPS, CPU assignments, GPU assignments, etc so we can confirm that they are what they are supposed to be. You can paste that back here for us
