Median Finding Algorithm for Joint Databases

You are interested in analyzing some hard to obtain data from two
separate databases. Each database contains n numerical values - so
there are 2n values total- and you may assume that no two values are
the same. You'd like to determine the median of this set of 2n values,
which we will define here to be the nth smallest value.
However, the only way you can access these values is through queries to
the databases. In a single query you can specify a value k to one of
the two databases and the chose database will return the k-th smallest
value that it contains.
Since queries are expensive you would like to compute the median using
as few queries as possible.
Give an algorithm that fins the median value using at most O(logn)
queries.