chose one option and give an external reason for it: e.g. "I chose pizza over pasta, because I had pasta yesterday". (I helps if you have a small list of bogus reasons prepared in advance: the weather, the price, what you did before, etc)
2: when they come back with the other choice, ask them why their choice.
3: randomly accept their choice, saying you understand or insist a bit on yours (2/2)