You were right. I do have the double mutant. Unfortunately, I don't
have much information about it and attempts to find out more haven't
been answered. Here's what I do know:
In C.elegans I believe, the double mutant appeared to be brighter than
the original GFP or the ind. mutants alone, when driven under the
unc-25 and vab-3 promoters.
They have broader emission and excitation spectra (rhodoamine filters
still show flourescence, although not as bright as fluor.).