no buffer to flush the error output in ?

[Phil Estival]

[[info:elisp#Synchronous Processes]] says

  You can’t directly specify a buffer to put the
  error output in; that is too difficult to
  implement.

Can someone explain why please ?

  But you can achieve this result by
  sending the error output to a temporary file and
  then inserting the file into a buffer when the
  subprocess finishes.

Only when it finishes? But there must certainly exist a way
to have stdout/err be separated buffered streams... right ?

Phil