title: Is it safe to run two instances of `stack` at the same time?
body: <p>See title.</p>
  
  <p>It would be nice if I could run multiple instances of stack at the same time. It would allow for some nice parallelisation.
  I do <em>not</em> know which commands I want to run beforehand, so I can not just merge the commands and have stack figure out how to do it in parallel.</p>
  
  <p>If this is not possible. Is it in the scope of Stack?</p>