Yes there is.
Select one of the slabs, and while in the slab tool, click on the edge of the slab. In the pop up menu, you will see an icon with a slab and a plus sign. This means 'add to the slab'. Magic wand/space bar click the other slab and AC will trace it's outline and add it to the existing slab. Keep in mind that the traced slab will still be there, so you may need to delete it.
Play around with it. If you can't follow those (rushed) instructions, and need screenshots, please let me know.
Cheers,
Link.