[wppick]

So what I did was wrote out all 16 possible outcomes in an excel spreadsheet. I used T and F for tails and heads, because I'm used to reasoning in true or false. http://i.imgur.com/jKYSo3P.png. Then I noticed the two edge cases of: TTTT, and FFFF and came to the conclusion that is was not possible for them to agree on the same guess beforehand (i.e. Both guess tails or both guess heads). So then, by trial and error, I checked what would happen if one chose the same as their flip, and the other chose different from their flip. I bolded the case where it was valid to my rules, and deleted an invalid case http://i.imgur.com/E3jnIkd.png, and saw that it worked.