Splitting Already split rooms

I am trying to split room room 5 along the line that was already used to split 4 from 5 but the dotted border seems not to be allowed. Want to split room 5 at red circled point. Any advice?

Currently that is not possible, you can only split exterior walls.


Major limitation. Look forward to the enhancement. Hopefully soon.

Came here to look for an answer to the same question. My kitchen and living room flow together but Iā€™d like to be able to vacuum them independently. I can only split them diagonally.