To find out how many centimeters of yarn each skein contained, we need to divide the total length of yarn by the number of skeins.
Harper ordered 2 skeins of yarn totaling 323.4 centimeters. So, we perform the following calculation:
\[ \text{Length of one skein} = \frac{\text{Total length of yarn}}{\text{Number of skeins}} = \frac{323.4 \text{ cm}}{2} \]
Calculating this gives:
\[ \frac{323.4}{2} = 161.7 \text{ cm} \]
Thus, each skein of yarn contained 161.7 centimeters.