I feel that we should look at adding more keys inboard rather than outboard of the core layout of the familiar Atreus, and that as you suggest, putting extra keys between the two halves might be a desirable way to achieve this, although to a greater extent.
The problem with the Atreus 62 is that it has puts an additional column on each side and - if we wanted the two halves of the layout as far apart as they will go - displace the home position of each pinky one key width inboard.
I think we should aim for widest spacing of the two hands. This either maximises the ergonomic benefits of the angled columns, or allow the columns to be less angled but with the same ergonomic benefit. To demonstrate, if you put your hands together on your desk (or lap) and then move them apart, your forearms will naturally rotate outwards. If you imagine the two halves of the Atreus were separated by the width of the laptop, each half can probably also be rotated a few degrees so the columns are more closely aligned along the notebook, which will marginally decrease the total forward to rear dimension of the keyboard.
For this reason I prefer a modification of the Atreus58, although the designer seems to have made a mess of the thumb cluster. I would extend the two-key columns starting with ‘Enter’ and ‘Space’ in the picture into full height 5 key inner columns. Essentially it would have superficially the same arrangement that you suggest consisting of six columns of five keys plus an inboard column of 2 keys to give 64 keys total, but a columnar stagger that places the home position of the pinkies on the outer column for widest typing stance. The space between the two halves could perhaps house an arrow cluster?