Moving this to the ''Venting, Chimneys' etc forum..
You need to make sure you can shut off either stove completely.. you need to have all the chimney draft going to the ONE stove that is in operation, and not have any air from the other stove going up the chimney.
When the stoker is running, this should not be an issue, the Mark II is an airtight stove, just shut the doors and spinner knob.. but there is the slight air gap at the top and bottom of the glass in the door.. probablly not a big problem.
But when burning the MarkII and not the stoker,, sealing the stoker from allowing air into the chimney is a bit more involved,, you need to cover the inlet to the combustion fan, probably just use the existing flapper and cover the inlet completely. But air can also get in through the hopper, so you must keep the cover on the hopper and make sure it fits tight. And then there is the same air gaps at the top and bottom of the door glass.
It probably would be best to have just one stove hooked up at a time, have the unused stove's flue pipe disconnected but already sized and fit so you can swap it into the chimney in a few minutes..
Unless you have a really strong-drawing chimney, you may be creating a low draft situation with the tee'd together flues..