This is how I calibrate my dual extruder heads. As I make them they are not a precise distance and need calibration. it has the 2 stl files to print the calibration and 2 PDF documents with instructions and the calibration sheet to calibrate your printer. And if you prefer its also shared on https://www.youmagine.com/designs/redsalamanders-dual-print-head-calibration-kit
http://www.thingiverse.com/thing:1009171