One way of doing it is to compute standard deviation for each film (as e.g. in http://stats.stackexchange.com/a/23254/6552https://stats.stackexchange.com/a/23254/6552) and average it over films (perhaps using the numbers of voters as the weights).
Other approach is measure average correlation (Pearson or Spearman) between their rates. Advantage (or disadvantage, depending what you want to do) of this approach is that it excludes simple shifts. E.g. if one person likes the same films as the other, but just casts 1 mark lower, it means that they agree.