Bathroom vent where there already is one?
I would not recommend this. If one fan is turned on and the other isn't, the fan that is on could send humid air backwards through the other fan (unless it has a working non-return valve).

I would advise either ducting this new fan completely separately, or instead getting an inline fan with a Y-piece to allow it to vent from two seperate grilles. See hvacquick.com for this example:



NB This is not a recommendation of a particular product/supplier/manufacturer