It is not possible to join slabs for now. What it is possible is adding boundaries to current slabs, but that can only be done if they are in the same plane. In this case, I think the best option is to leave them as separate slabs for now. We already have in our to-do list a feature to join slabs, so we will keep you updated about it.
Hi @user28 I let you know that we have implemented this feature in VisualARQ 3. Slabs now calculate their intersection when they overlapp or touch each other, and remove the intersection lines if they have the same attributes. This also applies to tilted slabs: