I replaced another worn out inline extractor fan yesterday which was as usual a straight forward job in itself, but the original vent tubing installed 15 years ago by the house builder was very unsatisfactory.
The 100mm extractor tubing from two separate bathrooms was joined together with T junction which then continued to to an outside vent via 9 metres of flexi 100mm tube. I suspect the damp air from one bathroom was partly being blown into the 2nd bathroom! Interestingly the home owner did have damp problems in the bathroom which could have been caused by the broken extractor anyway but I suspect having the two extract vents joined would not have helped either. Also the vent tubing was partly crushed in a couple of places as well.
The solution to this problem will be to re route at least one of the vent tubes to a new outside grill through the soffit board. I’ll offer the home owner a price for this.
I would not recommend joining two extract vent tubes into one single outlet tube unless exceptionally it is carefully design with components designed for such a scenario.
On a related subject, a few weeks ago I came across another bathroom with mold problems. This one had a lovely power shower installed but no extractor fan! Admittedly there was a window but who is going to open a window wide in the winter while having a hot shower and stand in front of it.
Consider the cost benefits over time if proper ventilation is installed in the first place in a well used shower/bathroom. If bathroom decoration and tiling is completely ruined by damp and mold then redecoration would be necessary far more frequently e.g every few years instead of 10 years or more.
A professional electrician will be able to advise on a proper solution for ventilation. It may not seem cheap at the time, but weigh it up against the cost of redecoration or even new bathroom tiling much earlier than normally expected.