Something like this. But how do I join the two separate meshes together so I can delete the face I have selected in the picture to form a corridor between the two boxes?
Boolean would do it, make sure rotation and scale is applied on both objects before proceeding.
Add a boolean modifier to the outer box and select the inner box as the target and “difference” as the operation, make sure the corridor extends slightly out of the outer cube before applying. After applying, you can delete the inner cube and fix the topology on the outer cube with the knife tool.
Here is an example .blend. Top is before boolean, bottom one is after boolean and fixing the ngon with the knife tool.
Bool.blend (444 KB)
Thank you! That worked well!