My favorite CycloComputer is the Campagnolo one. I like the integration with the shifters. It can tell me my cadence, speed what gear I'm in. Does everything I need.

If you want a computer for the back person on a tandem, why not just get two computers one for the front and one for the back. Hook one sensor to each wheel. Person in teh back gets hooked up to the back wheel and person in front to the front wheel.