C++ sort either A or B and sort in reverse order in case of the other one. By doing so, we guarantee that the added pair were optimized.

string twoArrays(int k, vector A, vector B) {

sort(A.begin(), A.end(), greater<int>());
sort(B.begin(), B.end());

int n = A.size();

for(int i = 0; i < n; ++i) {
    if(k > A[i] + B[i]) {
        return "NO";
    }
}
return "YES";

}