I have a triple shifter working with my double crank. I set it up, like TO11MTM said, with lots of extra trim adjustments. I can now get to any rear cog from the either front ring without any kind of rub whatsoever. Its quite nice.
There may be a differance between shifters though, I use 105 Brifters, they might be a bit differant than Sora. Im not sure there.
About the 7-8 speed compatibility. You could always throw on a new cassette and call it a day.
-Jerry