It is not a common setup and may be prohibited.
It *is* done though but the results are unpredictable.
Each fan normally has some kind of "flapper" valve
inside to prevent backdrafting. You could see if
that is present and working.

Thanks. Will look into this. Most likely there is no flapper. These are
Broan fans if that means anything to you. Perhaps, the flapper, if present,
doesn't work as well for the master bath's fan because it is mounted
sideways in a skylight's opening, rather than on the ceiling. It could be
that, when idle, the flow from the 2nd fan, although powerful enough to make
the blades spin, still isn't powerful enough to close the flapper, but I'd
bet that the flapper is just absent. Will see what I can find.

In the end, I suspect that it won't be a big problem;
see what your "steam test" shows.

But you paid for it; it's your call...

Thanks. I will see what happens with the steam test. By the way, cross
flow from the active to the idle fan, although it obviously exists, seems
relatively weak when I hold my hand near the idle fan. I will try the steam
test and see if this issue really is a problem or not.

By the way, even though I just spent close to $300 extending the exhausts,
I'm skeptical that these fans, even when properly installed, really help
that much. These Broan fans, even though they SOUND like they work, each
can BARELY cause a sheet or two of toilet paper to defy gravity if I place a
piece or two across the opening. In other words, I'm skeptical that these
fans really suck enough air to get rid of anywhere near ALL the excess
moisture/humidity when taking a shower. Perhaps it's only 25% of the excess
moisture/humidity tops.

Jeff