I would make a section of the dividing wall doubled, with a 12" to 16" space between them, and run the heat riser up in that space. Then I would connect to a cavity in the mass under the bed for the hot gases to flow into, and from the bottom of that cavity connect to the chimney rising to the roof.
That is the bare outline; the dimensions would depend on the details of your spaces, any doorway between them, and your heating requirements. As you appear to be in the UK where winters are usually milder than in the northern US, and the spaces you want to heat are relatively small, I think a 6" system would work fine for you. For instant radiation, I would mount a metal plate in the wall of the cavity, which would also serve as an access port for inspection and maintenance of the riser. You could put a plate in each room if instant radiation is important in both. Whether you build a conventional J-tube RMH core or a batch box is up to you; in either case, you can use the information at batchrocket.eu
for component sizing and cavity layout
. It is generally thought that a 6" batch box gives equivalent heat output to an 8" J-tube, so you could even make a smaller size batch box, maybe a 5" system, and get good results. If you have the room, a 6" system would guarantee that you had enough capacity, as if it is oversized, you simply run it less often.
If you give more details on the heating loads and the space layout and building materials, we can give you more detailed advice.