The Manchester is a convective stove so maybe that would work better than a steel /cast or soap stone stove. I don't know.
There are different variants of cast iron stoves out there. The fully cast iron ones certainly produce a lot of radiant heat. Newer models, however, often have a steel firebox and only a cast iron outside shell. Those are usually more convective type stoves. The PE T-line would be an example. From the stoves you may consider the Jotul F55 falls in that category. With 14" side clearance with double wall pipe it is certainly a more convective stove although the Manchester does not seem far off either.
The idea of putting a second insert/stove in the other fireplace is worthwhile considering. You have not posted a floorplan yet but having another stove at the other end of the house would probably give you a more even heat. However, tending two stoves can be pretty time-consuming. To get the warm air out of that room you can try the following trick:
Place a small fan on the floor opposite the stove room. Blow cold air on low into the stove room. Warm air will rush out along the ceiling to replace the cold air that has been moved by the fan. The location/room the fan sits usually warms up by several degrees within 30 minutes. That trick is based on the principle that cold air is denser and therefore easier to move than warm air. You could already try it with your current insert.