Upvoted for (after ignoring #include <bits/stdc++.h>!)

swap( a[0], b[0] );

I personally did it this way:

cout << b.at(0) << a.substr(1) << ' ';
cout << a.at(0) << b.substr(1) << endl;

I know that mine would throw out_of_range but what happens, with your solution, if a or b is empty?

By the way, for anyone wondering: string::size() and string::length() are exactly the same function! Which is pretty uncommon ...