I think I missed the point about the master switch ;-) Sounds a bit like
overkill as the two zone programmers can be co-located anyway.

Co-location is impossible. They are programmable room thermostats. They
won't sense the right temperature if they are in the wrong room. I suppose
you could use a multi-channel programmer and straight room stats. However,
this would cause many disadvantages. Firstly, this system doesn't allow
different temperatures at different times of day. Secondly, manual override
(i.e. boost/advance) buttons will be in a dark cupboard under the sink with
bottles of bleach in the way, rather than on a convenient well lit nearby
wall.

Christian.