It is very difficult (in fact impossible) to find an image of a double encased double overlay on the web!!
This is a nice example of an encased double overlay http://www.lhselmanltd.com/auc25/43.html
As an idea of what they might look like, here is a picture of a Perthsire which I think is a double encased double flash overlay - although I could be wrong... http://www.lhselmanltd.com/auc27/48.html
For Karelm and aa , flash overlay differs in that instead of using a glass cup, the hot weight is rolled in couloured glass powder and reheated until a good layer is applied. This can then be removed by cutting and facetting. The picture - one of the Perthshire hollow blown series, has been given a flash overlay, facetted then encased , then given another flasj overlay and faceted again.
As Kevin indicates, Perthshire did a number of d.e.d.a weights. See http://www.pcatx.org/interest.htm
(Paperweight Collectors Association of Texas). Just over half way down is a reference to a talk given about Perthshires final year. This refers to a CD they have that shows 52 Perthshire double-encased double-overlay paperweights. Now if anyone has any contacts there to get a view.
And as aa suggests - it is all very difficult since every process is doubled, the weight has to be heated and cooled more than once so additional fracture/ennealing problems, and given wastage and the percentage of weights have faults, the amount of work that could be lost.