I'm not sure if you can achieve it by setting a larger crop shape in combination with the .SplitRegionHorizontally/vertically methods.
RevitAPI.chm ViewCropRegionShapeManager.SetCropShape:
"Depending on the shape of the argument, view's crop is set to be either rectangular or non-rectangular. If the crop is set to be rectangular and it is also split, then the multiple view regions will be displayed for the view, with the same proportions as the split prior to the change, but adjusted to the new rectangular shape."
No sure if it means what I believe it means. I think it means if you create a larger rectangle you create a larger offset extent for the split.
ViewCropRegionShapeManager.SplitRegionHorizontally/vertically
This controls the portion assignments of the slit to left/right (0 to 1).
One thing noted recently on this forum was the fact ViewCropRegionShapeManager.SetCropShape can't be used to set a rectangular shape. However perhaps that is permitted if the crop has splits? If so you would have to create the splits first.
That is the shape of my argument anyway.