Ok - I do have it and I installed a second instance called "tor2".
I can control the first instance of Tor as usual with arm but how can I now control the running of "tor2" with arm?
When calling arm with "sudo -u _tor-tor2 arm"
don't run arm as the tor user, Roger tells you why: https://lists.torproject.org/pipermail/tor-relays/2016-May/009259.html
since your second tor instance waits on 9052 for incoming controlport connections you have to tell arm about it
arm -i 9052