there a few case that have a vertical mutly slot. coolermaster had one(Cooler Master SL600M) , lian li 011 i think has one. posably there a bolt on one now that dose do 2 or more. thrmaltake core p case can do it i think.
lian li 011 evo can mount 2 gpus in different spots
there are gpu fan mounts that allows you to mout a gpu to a fan from mnpc tech
witch the the best case / option
might have been some case from ces that had some options too if i rember but dint think there out yet...
can also diy mount a gpu as well.
Thermaltake CTE C750 has a rotatable pcie slot
lian li v3000 plus i no can do 2 gpus but cost like $900...
there saposably an lian li 011 evo xl coming at some point.