Roy
I thought the reason for these cutters was to do the cut in one go..hence min max material thickness.
The pressure build up on the very fine edge you are trying to achieve by multiple passes,results in either the fine edge collapsing or you take too much off, and the finished joint ends as a miss match.
Making this joint on two mating parts done in two different set up operations, isn't IMO as easy as you make out.
The edge of the joint tapers to nothing on both parts. If it didn't then you would get a step, no matter how small and to fix this you would have to round it off with abrasive, you would also get an out of square box when assembled.
The other point is, that to achieve a zero thickness edge, you need a router table set up that when using the sled, gives you, no movement what so ever in the track.
The parallelism of the track to the fence has to be accurate within seconds of a degree, or over a long distance the mismatch of the two joining pieces can become so large that the two parts will not go together.
If they do then the four parts assembled and cut into an oblong shape, produce a tapered oblong.
That is if you are using the fence as the stop for the depth of cut.
Granted on smaller lengths this error is reduced.
I do not want to put anyone off from using these cutters.
But there are big limitations when used in a home made router table set up and you should be aware of the down falls.
You may over come some of these issues if you invest in some incra gear.
Making a box from mitred stock is not an easy task IMO, even using a table saw and getting the angle correct, then making a jig to take the feathered edge, to machine the opposite edge, and to a set dimension when assembled, has it's problems, and isn't as straight forward as members made out in another thread.
Mikee
) That is when you use it for both fronts, side and bottom. 
