You can always use the "Crop" feature. It is not the same I know, but it works sufficiently well.
Render like the top 60% today and when finished, save the file and close.
Tomorrow open the file, adjust the Crop to the Bottom 60% and render and then put them together in Photoshop or whatever your editing software is.
Now why 60%? Because the camera sees the edges as the end of the lens and therefore distorts the middle cropped edge. It also distorts the outer edges, but they do not matter in this case. So by rendering 60% on both Top and Bottom you get some overlap into the area not as badly distorted by the edge of the camera. Sometimes I have to even go to 65 or 70%.
Again, it is not the same, but it will accomplish what you want it to.