I'm leaning towards buying another indoor unit and installing the new 'system' in the den.
(Where it can help out that poorly insulated room).
If for some reason, I can't get it to work in the den, a second indoor unit would give us a
complete
set of spare parts for the existing main-house system. That could be very useful in the event of a lightning hit.
Plus, it would give us a spare remote control too. These things are hard to control without that remote.
Actually a single remote could control multiple systems, by setting the
indoor units to used their on-board thermistors.
(Instead of using the thermistor built into the remote controller)
Then, the remote would used for all other controlling functions as needed.
(Mostly Set points & operating modes).
Cheers,
Rich