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My probel is that I have two trackers to coordinate. One is
our roundup instance (roundup in the following), the other
is some other bugtracker used by other projects we depend
upon (upstream).

What I get is a report of some problem in roundup. I look at
that and come to the conclusion that this is actually a
problem with one of the libraries and report that problem on
upstream.

I can request updates on upstrea to be reported to a
email-address I give. If roundup was able to understand
issueX@roundup.instan.ce then I could enter that into
upstream and thus get a trace of the upstream-activity in my
own roundup tracker. I cannot specify a prefix that the
upstream tracker will prepend (like [issueX]), so I cannot
get roundup to accept mails from upstream without some
serious hacking of my mail configuration:-)

The outgoing mails are not that bad: You can have roundup
send mails to issueX@downstream.org allready, so that way
thwere's no problem with collaboration. I'd still prefer
issueX@roundup.instan.ce though, but that's just a matter of
taste.

When do I want to send the same message to different issues?

One example is when several people report the same bug und I
want to mark all but one to be duplicates of the one I want
to keep open. CCing that to all the issues about to get
closed seems like a very clean solution to me. Allowing more
then one issue in the Subject-Prefix should work too.