Gemara in menachos 37a-b discusses how much you need to give for pidyon haben if they are a bechor. (it is also the gemara with which head you wear tefillin on.)
http://hebrewbooks.org/shas.aspx?mesechta=30&daf=37&format=pdf
And tosfos there quotes the medrash with shlomo about the guy that ashmedai brought with two heads who had kids with two heads and they wanted double yerusha. I don’t know where it is.