A matter of nomenclature, as I understand it. A knife is not double edged unless both edges are the same length or nearly so (3/4 or more is a common place to draw the line). What you have been talking about is a sharpened false edge, not a true double edge.
The problem with a sharpened false edge or a double edge on a folder, as Tommi so astutely pointed out, is that the secondary edge has to be recessed in the scales, or the damn thing will cut everything it touches. A sheath would be mandatory for an exposed sharp secondary edge.
As a matter of fact, the BM 875 (Sentinel) has a non-sharpened false edge which extends three inches back from the point of the four inch blade (at least on the two specimens I just measured). This would barely qualify it as a double edged blade should the false edge be sharpened. As it happens, the first 1 and 3/4" of the false edge (measured from the tip) is recessed into the scales, and could be sharpened with reasonable safety. Beyond that, the exposed sharpened secondary edge would represent a hazard to the operator.
I think that you might reconsider your project; just what do you hope to achieve with a folder of this nature? I fail to see any merit to this sort of knife at all.
Walt