jmc

There are four possible ways for the cubes to land: two red faces up, the first with a red face and the second with a white face up, the first with a white face and the second with a red face up, and two white faces up. So, there are two ways that the cubes can land such that there is a red face up and a white face up. The probability that we get a red face up is $\frac{3}{6}=\frac{1}{2}$. Similarly, the probability that we get a white face up is $\frac{1}{2}$. Thus, the probability that the cubes land such that there is one red face and one white face up is $2\cdot \frac{1}{2}\cdot \frac{1}{2}=\boxed{\frac{1}{2}}$.

Alternate solution: After you roll the first cube, the second one must be the other color, which occurs with probability $\frac{3}{6}=\boxed{\frac{1}{2}}$.