1) Make a copy of your D3 install
2) Create a shortcut to Diablo III.exe.
3) Right Click Properties in the target field add -launch to the end, it should look something like this. "C:\Games\Diablo III - Copy\Diablo III.exe" -launch
You will need to do this for every instance of D3 you want to run at a time, now you don't have to copy the D3 install, but if you don't the load times will be much longer.