Search for "i2c nfc". These parts are larger than 2mm and the design problems are primarily software. Also, offtopic. This is the whole idea of the Other Halves. Yes, this is the whole idea of I2C, also: google. Stackable OH's is the worst idea ever, get over it. Yes it is possible, just as possible as gluing you phone to a rock and throwing it in the sea. Still, terrible idea. And offtopic.