Answer would be O(nlgn)
In order to find middle of union array atleast half of both the arrays should be compared and we can do this by using binary search on both the arrays, so three cases arrives,
1) both m/2 and n/2 array are covered then we will find the middle element (or)
2) array 1 is totally over (or)
3) array 2nd is totally over
In all these three cases binary search will take O(nlgn)