Two batteries design tends to occur more frequently, to allow non rectangle battery...
This is not the only reason. The other reason is shipping: You can ship up to 100Wh, but only up to 20Wh per battery / cell.
So that's a workaround, as each one only has 18Wh.
The GPD Win or GPD Pocket could usually not be shipped with a normal shipping service, as it has more than 20Wh in one cell (though no one really cares about that).