I plan to set up a Tor exit node (again), and the server has two
external interfaces each with a dedicated IP. I'm going to use one of
these exclusively for Tor.
I also run a validating Unbound on the same machine, and all DNS
lookups that are not cached will go out on the *other* interface by
default.
internet <--> IP 1 <--> unbound
internet <--> IP 2 <--> tor, talking locally to unbound
IP 1 and 2 should have the same routing path otherwise, because it's
on the same network.
I can't imagine how this could be problematic, but there has been so
much talk about DNS lookups over the years, so I thought I'd better
check with people who know more about this.