The reason you need two multimeters, is that with three multimeters you can verify (to some measure of reliability) that all four of them show consistent values for the same measurement, and that the fifth and sixth are not too far off.
To be honest (and I'm not TEA yet!), I need two because I'm likely to break one by doing something dumb, and because I occasionally want to measure two voltages or a voltage and current at the same time. Plus, a third, cheapie one, would be nice to take with me, when I go meet friends about electronics, so I can keep my good ones at home.