Considering HRV Solutions for Steam Removal in 2 Adjacent Bathrooms

air

Hi there,
Ok, there are two bathrooms with showers on the 1st floor of my house - 1 en suite and 1 main bathroom.
Until now steam has been removed using the old open the window method, which has a number of obvious disadvantages.

As a result I've had a quick look at HRV options and have come up with the following two solutions:
1. A cheap whole house HRV system which I could put in the attic and easily duct from over each shower to outside etc.
2. These single room MHRV systems which are quite cheap on ebay also.

Now I read criticisms of the single room systems (albeit from competitors that didnt offer them) which seemed to be primarily based on the idea that the prevailing wind speeds would overcome the fans I think.
Anyway the multi room system would suit me better anyway because it's designed to be ducted and I could do that end of things easily.
It also has the advantage of not requiring me to bore holes in the outside walls.

So, my basic idea is to plant one of these in the attic and duct it up, and possibly put humidity sensors in the ceilings beside each duct point to run the system.
I have a number of questions:
1. Am I wasting my time, should I go for simple ducted extractors?
2. Are there baffles / diverters available so that I could use a bit of logic to ensure that only the bathroom with steam is ventilated? Links appreciated
3. Can someone recommend suitable humidity sensors? again Links appreciated

Another possible benefit of the small HRV system is that I could also feasibly duct the remaining rooms / bedrooms in the house using this and then block the fixed wall vents in those rooms which would presumably help the efficiency of the house further.