You definitely want separate halves, or a at least a hole between them. The grid forces quite aggressive ulnar deviation otherwise, if you touch type using the standard technique. It's not that bad on a low-profile keyboard, such as the TypeMatrix, but sucks with more profiled, tall keys.
There are plenty of POS matrix keyboards. Usually from Access-IS or Tipro. The latter usually support daisy-chaining. There's been the Humble Hacker project and later
GH36x2 as well, but it seems samwisekoi has abandoned it…?
I don't see the point of not going the whole way to a columnar layout with dedicated thumb clusters though, unless you're on budget and want to reuse existing parts.