Funny timing, I just posted about this in my blog last night. I'm doing the exact same thing. The only way I could figure out to do it was to manually set it up and save it as a custom layout.
I opened up 4 tables tiled, then another 4 and manually positioned & sized them over top of the previous ones... so I had 4 tiles each 2 deep. Then I went View --> Manage Layouts --> Save As and checked the "apply successive" option. (I'm not 100% sure what that option does though).
A couple of notes about this:
1) It seems to open up tables by position first, so your first 2 tables will be stacked in slot 1, then the third table that opens will be in slot 2, fourth table will open in slot 2, fifth table will open in slot 3, sixth table will open in slot 3, etc...
2) It only knows what to do for as many tables as you have set when you save it. So if you save your configuration with 4 tiles x 2 deep and then open up a 9th table - it won't be in one of the slots...
Hope this helps.