Take two styles that have enough in common and adulterate prior to fermentation? The main things in common I'm thinking are general kettle hopping, IBUs and gravity.
I'm thinking something like:
IPA, Amber Ale
Helles, Dark Lager (vienna, dunkel, etc.)
So I'm thinking it could go like this:
mash to make 10 gals of the lighter color style.
meanwhile make a mini mash of roasted or darker grains with a little base malt.
drain the big mash tun into boil kettle and make beer no sparge style.
meanwhile dump the mini mash into the big mash tun and "sparge" what is left in there into a small kettle and briefly boil this.
at the end split the 10 gal of light color wort and add the dark wort to half of it.
ferment separately, dry hop separately etc.