If the rooms are to be the same size, a straight wall can be installed so that the room at the top of the plan has two windows and the room at the bottom of the plan has one. The middle door will be closed. Whether the room at the bottom of the plan is bright enough can perhaps be tested (how exactly, the location would need to be known). Are there roller shutters? Depending on that, the lower room could then receive a larger/new window. The minimal solution (for two equally sized rooms) requires removing the partition wall, installing a new partition wall, closing the door (i.e., frame out, brick up ...). The costs depend on that (DIY yes/no; other conditions and later renovation effort, floors ...). Or am I completely wrong? What is wrong with this solution?