JCMT information from Gerald Moriarty-Schieven:

Joint (multiple queues) proposals are allowed, but each TAG will award
time separately, only from its own queue.  Aside from noting joint
proposals at the itac meeting, usually no special considerations are
needed at the itac meeting.

The ITAC consists of one member (usually the chair) of each of the
national TAGs, plus one more from the UK.

After I receive the proposals (including UH), I generate a source list of
all requested sources from all proposals, and look for duplicated sources.
I then check to see if the duplicated sources will be done with the same
or different instrument.  Where there is substantial overlap, I will
inform the TAG chairs.  If the overlapping proposals are in the same
queue, I let the national queue deal with it.  If from different queues,
then the ITAC will deal with it during their meeting.  This is usually
done on a case-by-case basis.

E.g. with the case Henry referred to, the ctag just made the two groups
collaborate.  Another case, the GRB Dutch/Canadian projects were told that
both rival groups would be given the data equally, and could publish
separately.  (Now both groups are collaborating on the JCMT project,
asking for time jointly from the NL/CN/UK queues.)  Sometimes the itac
merely suggests that the two groups (if awarded time separately by
different queues) collaborate on the overlap sources, but doesn't enforce
it.

Each TAG has its own policies on large or legacy-type programs, and the
time awarded to these (e.g. SHADES) is deducted from the national queue.
The UK and CN TAGs allow several pages for the scientific justification
for such large-scale proposals, but I'm not sure if NL does the same.

As you know, there is still debate on how large-scale scuba2/harp surveys
will be conducted.