Right, I'm going to make a cods of describing this, but here goes.

Two small bathrooms, one above the other, in a dormer bungalow under
redevelopment. Both need extractor fans (the lower one has a window
that was once external but now opens into an entrance hall).

The upper bathroom will have an extractor in the ceiling, which will
duct to a vent chimney thing that replaces one of the tiles in the
sloping roof above the ceiling.

It would be possible to extract the lower bathroom by means of a
ceiling fan that ducts up into the eaves space, then up inside the
sloping roof, joining the same vent chimney thing Above the upper
bathroom ceiling by means of some kind of T.

Is this a good idea, or might I end up blowing wet air from one
bathroom into the other, or causing water to condense and run down the
duct into the other bathroom? Would it be better simply to put in a
second vent chimney thing lower down, in the eaves, to vent the lower
bathroom independently?

Most importantly, has anyone here actually combined vents like this,
or seen it done? Is it an orthodox thing to do, or not?

Cheers
Richard