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.

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