It's probably easiest to use systemd. Take a look at the service configuration file located at
/lib/systemd/system/tor@.service. , '%i' is a placeholder which will be replaced with the name of the service. The systemd service has also some nice hardening features enabled by default.
Here are the instructions assuming the service is called tor1
- create user _tor-tor1
- create group _tor-tor1
- start service
systemctl start tor@tor1 (creates a custom torrc)
- adjust torrc at /etc/tor/instances/tor1/torrc
- enable service (start automatically on boot)
systemctl enable tor@tor1
- restart service
systemctl restart tor@tor1
You can use OutboundBindAddress to set the address from which packages are sent and you'll also have to make sure you use a different port.
(Debian unfortunately didn't have systemd when I last set up a tor relay, so the instructions may not be 100% accurate. Please let me know if that's the case.)