Since my last post I have flattened the two top slabs. Flattening the slabs involves using hand planes to plane away high spots, slowly bringing the entire surface into the same plane. Once flat a marking gauge is referenced off the flat surface and run around the slab to mark a line parallel to the flat face. Once again hand planes are used to remove material until the gauge line is reached. This process is very effective but very slow. But, when working with such large boards the methods for milling are very limited. It's not uncommon to remove a 1/2" of material from the entire face to get to flat. The flattening process easily consumes a large portion of the total build time.
With the top flat I turned my attention towards the battens that will attach the two tops. I decided to use a sliding dovetail joint to attach the two tops. This solves a couple problems. Firstly, it'll keep the table flat over time and second it'll allow the top to expand and contract with the seasons as wood is wont to do. Not to mention it's pretty damn cool.