In the sub-script that you call using RUNAS,
  • Use RedirectOutput() to redirect to a new file - if the output file is already open for output in a parent script you won't be able to open it again.
  • Make sure that you are in the correct folder. When you execute under RUNAS your context will change, and you may end up in (say) C:\Windows\System32 so use the KiXtart "GO" and "CD" command to ensure that you are in the right folder before you issue the RedirectOutput()

If you want to append to the same file you need to close the RedirectOutput() in the parent before calling the script then re-open it afterwards.