Two bathroom extractor fans one external vent

Ive been thinking of doing some ceiling vents in the bathroom/ensuite myself (none currently).

What you're saying is you're going to go up from the top of the ceiling upwards to the underside of the roof.
Aren't they for ventilation within the attic, maybe no problem using one if you had other vent tiles and better than venting moist air directly up there but with no other vent tiles,wouldnt be considering it myself unless it was for that purpose?
Also, wont any moist air that condenses internally drain back down to the vent?

Im planning to bring the vents in solid pipe up into the attic and then connect both vents via some flaps that prevent back flow from the other room when only one vent is in use, do this via a length of plastic pipe maybe 50mm diameter and in such a way it slopes down to the vent, which I plan to do on the soffit.
The solid ducting for vents is pricey per metre, so going to connect into a generic lenght of drain pipe for the long runs and use adapters to connect vent fittings to and from it.

Also, Id be insulating the pipe so condensation didnt occur on the outside and drip into the attic under certain conditions, although I think mostly the condensation would occur internally in the pipe surfaces, hence having a fall on it so any liquid could drain out.

On top of this, I think flexible tubes are pretty rubbish for expelling air, better than nothing but they can be damaged with small holes even when you get them and its easy to accidentally make small holes.