I think Jolliff and VPI were proven non compatable by breeding together so pffspring from that would have been double hets. I do not know if TSK and VPI have been crossed but since the commonly held opinion is that they are also non compatable I would guess it too has been done, again resulting in double het progeny.

If you bred double hets togethere to get double homozygous you might get a more stable black/silver snake (i.e. less brown out) but I do not hink you can say with out actully doing it.