I just created a tool to help with calibrating the XY offset for the BigBox, though it's equally applicable to all dual head printing. http://www.thingiverse.com/thing:1456697 I recommend you use Greg's "pin" method to get things close, but for me I wanted to get this super accurate, and the pin method is prone to parallax error. Once you've used the pin method, download the base, 0.2, 0.1, and 0.05 things and, using S3D, composite first 0.2 and base using the dual extrusion wizard. Once printed, selected the best aligned blobs and then figure out from the model shown in S3D how many blobs away from what should have been aligned your aligned blobs are. Nudge your M218 settings in the tool change code in respect of your findings, and then print the 0.1 and again find the closest fit and adjust. Finally do the 0.05. At the end of the process, you should be within 50 microns of accuracy on your XY offset and your dual prints should be bang on.