Finding median of two separate databases.

[four80eastfan]

I'm not too sure how to tackle this question:

Two databases, each containing n elements (for a total of 2n elements, with no two elements being the same). Want to find the median (n-th smallest value). The only way you can access these values is through queries to the databases. For each query, you can only specify a parameter k and the chosen DB will return its k-th smallest value. Give an algorithm that finds the median using at most O(log n) queries.

Thanks in advance,

Matt