If they invent some way to disperse the heat effectively from two GPUs on a single PCB I'm all ears.
That would effectively need a single PCB that is double thickness and has top and bottom cooling that is identical.
You're right though, I've yet to see one that disperses the heat evenly and you always end up with one hot core.
Which kind of defeats the purpose, given actual SLI is pretty much the same and costs loads less and is usually faster as the cores are far more beefy.