N8N wrote:

What I'm thinking of doing is just not talking to the guy and getting some =
proper 3" flex duct, and a 2x3" to single 4" y adapter and hooking things u=
p that way. It'll still look like a knot in a canine's genitalia but at le=
ast both fans would be vented to the outside.

Questions: would this meet code? Is there any downside to doing it this wa=
y other than being ugly and messing up the attic space?

I can tell you about 10 years ago I was looking into using a common remote
extraction fan for a couple of bathrooms in a house we were builing. My builder
said the local code guys would not approve that and we ended up with bog
standard fans in each bathroom each going to their own stack.

Seemed really stupid to me at the time as there are millions of hotels with
common extraction fans - certainly they meet code, but it wasn't a fight I
wanted to have.

The problem with doing it on your own, even with backflow prevention, is that at
some point in the future when the house is up for sale, some inspector is going
to flag it and the owner is not going to be happy at having to deal with it to
make the sale go through.

I'd drop by your local code office and have a friendly conversation with them.
I've found they are far more willing to let you know what will pass in your area
up front, than they are after the fact.