That's not correct. I have a Garmin GPS175 (the GPS part of the GNC355) and I can send flight plans from Foreflight on my iPhone/iPad to the 175 via bluetooth, as
@Denok indicated. My friend with a GNX375 can do the same. The GNC355 (and the GTN650/750 series) are no different.
What I generally do is NOT hook up to the Skyview's WiFi, allow bluetooth to connect to the 175, send the plan to the 175, then connect to wifi and send the plan to the Skyview. Clunky. But doable all on the iPhone/iPad, without turning off either the Skyview HDX or the 175 - just use the settings in the i<device>.
But it certainly would be nice if Foreflight had the ability to choose between the two "panels", so you could do one or the other. I think the problem is Foreflight's, not Dynon's (or Garmin's). It can only send to whichever was the last thing it hooked up to.