The number of bevels has less to do with a knifes usefulness in carving wood than the total angle and the length of the bevels.
If a knife will be used for carving basswood, the bevel nearest the edge (if there are two) should be the longest.
Conversely, (now there is a four dollar word) if the knife will be used for harder wood or utility uses, then the bevel nearest the edge should be the shortest of the two.
lly, a single bevel that runs from the cutting edge to the heel of the blade is best for basswood. This is not always possible or plausable (not sure what that means, but is sounds good).
As far as rounding the transition between the two bevels, it is pretty much a matter of choice; because the secondary bevel is always a smaller angle than the cutting planes of the primary bevel. :P