Given that the jury is still out on the whole whisper quiet A/C setup, why would you even consider doing that at this time. Many like myself are putting blocks in between the whisper quiet units so we can at least sleep in a cool bedroom. Shared ducts? I would like to hear from a HVAC engineer on the whole concept. Forcing air out the supply ducts when there is large path through a second A/C unit that is not running makes little sense to me. The integrity of the ceiling ducts also comes into question when the duct is an inverted C that is not sealed to the top side of the ceiling panels.
I have even ordered another vent/register for the bedroom. Three supply and two returns does not seem very balanced, given that the return duct terminates at the shower sky light. Supply air into the bathroom and no return register to help return it when both BR doors are closed.
Don't get me wrong, we love our new 2017 BH3575el, however we are working through this issues, sealing gaps and holes with HVAC foil tape. Chris