Call me Ishmael, but I really have a very basic understanding of game theory. I know the prisoner's dilemma, and know a little Nash from A Beautiful Mind, but my basic understanding boils down to: do all you can to advance yourself; if that is not enough do what you can to fuck the other person over.
So let's talk about game theory.
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Always maximise expected utility; that is the cardinal rule.
My girlfriend and I were watching a Teaching Company series on game theory a few years ago, and it was really interesting, but we never quite finished it.
However, you must note that if there is some mechanism you can use in order to ensure neither of you testifies, then that is preferable to both prisoners testifying.