Redoing two rooms that were previously (prior owner) a garage and storage and are now a living room and office.
The office is a triangular room (12:12) with the roof deck having recently been spray foamed (thanks for all help).

Living room: 20 x 21 x 7 1/2 feet
Office: 20 x 17 x 8 feet

Currently they are served by the same heat pump but there has always been massive differences in temp between the two as they are sealed off from each other aside from the ductwork. No return ductwork. Air handler in the living room.

Wondering if a dual minisplit would be a good replacement here to keep the the rooms separate and managed as such.

Insulation is now better than it was since the R-11 cathedral insulation is now 3″ ccspf covered with somewhat compressed R-19 batts. R-11 walls are now R-15 Roxul with R-5 exterior xps.

Zone 4 (SW Missouri)

Concrete slab is uninsulated though and can get very cold around the exposed perimeter. This spring I plan to add R-5 xps around the outside to help. 
Had also thought about installing electric radiant flooring knowing it would be inefficient due to no underslab insulation but it has been pretty miserable on the feet in winters past. Due to ceiling height unable (or unwilling) to raise the floor any.