The Tor client itself does not generate any significant isp traffic when idle per instance. You may use something like wireshark to verify this. It is somewhat less load on the tor network (circuit creation/maintenance and overhead) to use fewer instances.
Assuming your two boxes are running on a home lan behind a nat to your isp... if you do not need to have two tor instances/boxes for political reasons, or really want to save that small amount of two instance bandwidth/load... you could simply run only the one relay instance and configure your other workstation box/apps to send through the socksport on the first box.
If you are newer to more complex configurations than just TBB, be careful to verify your configs and test things with wireshark, particularly that you have no non-tor traffic leaving your gateway, if that is your intention.
Whonix may give you some ideas too.
Thanks for trying out a relay.