Not sure this is possible. With Google it is possible because it’s not really two separate pages, just two steps on the same page. Your password field is just hidden from view in the first step, so we can still fill it.
With Yahoo, it is literally two separate pages. In the world of stateless HTTP we have no idea that you still want to fill step 2 after step 1.
There’s also something else that could ease this step, like shown here:
As you can see, I can either click the button at the end of the field or press the shortcut key to cycle between logins/passwords (like told on the thread above).
On the 1st image (with Yahoo), you can see the drop-down “Log in as”. As I clicked it, you can fill in with those quick options.
Notice there’s a magnifying glass on the right of the pop-up (of the second image). That happens when there are many logins, so we can type in to find them to fill in. It’s just a quicker way to access and auto-fill the field.
But that’s something for another thread. Sorry for getting off-topic. Just to make sure you understand.