How to Layer Slabs And Generall Questions

[Kouseki]

Hello everyone,

Im trying to achieve an effect similar to the one in the Blase Code Gallery "Blast-o-matic" (http://www.blastcode.com/gallery/sample.php?pid=109). I'm having the Hardest time trying to set up two slabs layered together. Can anyone help me out?

What I'm trying to do is to slowly obliterate a small cityscape ( 6-8 building). I'm having a hard time getting the buildings to slowly start crumbling instead of having the whole building break.

( Sorta like this: http://www.youtube.com/watch?v=03FdHZaTFtQ ).

I've played with the explosive and the Second Degree options in the slab. Does anyone have any suggestions? Thank you.