I can see an airlock type system using materials to make a second smaller partitioned area inside the first.
A casts ward on our area with B not invested. B invited in by A. A leaves. B casts ward on smaller area. Now neither is invested in the other's space, and it requires both people to approve of entry.
Otherwise, no, you can't have two wards on the same entryway, and anyone who is invested can let someone in/out, thus no single entry dial approval system.
Keep in mind the airlock area, if it has two exits (outside to airlock, then airlock into rest of building), would need two casters.