If you have two different wall constructions (e.g. brick and wood) in the same wall plane then you can define the secondary (less prevalent) construction using a sub-surface.

To do this you should navigate down to the surface level to draw the sub-surface (in the same way as drawing windows, holes and doors). You can set the sub-surface construction on the Construction tab at any level you wish (building, block, zone, surface) and the data will be inherited by levels below it.