I am installing extractor fans and 100mm ducting from two separate but
adjacent toilets. I would like to run the ducting through the loft space and
to open air using a single
outlet plate on the external wall face. This means I have to join the
ducting at some point inside before the cavity. My problem is I can't seem
to find an adaptor that will allow me to feed two into one. Does anyone know
of one, or the best way to make one up?

Many thanks.