TweenCurves accepts only integers as values of Number option.
TweenSurfaces accepts only integers as values of NumberOfSurfaces option.
If we could enter negative values, they would be interpreted as curves and surfaces drawn beyond the original objects (rather than between the original objects).
It would also be useful to have Offset option within both commands. The Offset option would work like the Offset command - the user would type a number or show a point where the new curve or surface would be created.