How Do I Manipulate Windows in AppleScript?

Questions & Suggestions
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Are you putting the script in an Execute an AppleScript action?

IAC, when you are trying to manipulate windows in AppleScript, you generally need to use "System Events" and the frontmost process. This revised script works for me:

use AppleScript version "2.4" -- Yosemite (10.10) or later
use scripting additions

set myApplication to name of current application
set screenWidth to 2560 --1920
set screenHeight to 1440

tell application "System Events"
  set frontProcess to first process whose frontmost is true
  tell frontProcess
    
    if (count of windows) ≤ 2 then
      set myWidth to screenWidth / 2
      tell window 1
        set position to {0, 0}
        set size to {myWidth, screenHeight}
      end tell
      
      tell window 2
        set position to {myWidth, 0}
        set size to {myWidth * 2, screenHeight}
      end tell
      
    end if
  end tell
end tell

BTW, when posting a script, please put it in a Code Block.

See WINDOWCOUNT function.