Merging both the register and the login button together is a bad idea. As it can result in a slight confusion as of what happens when the return key is pressed both on touch devices and on PCs, "Join us today" is to blame here. This can also result it accidental tap/click of the wrong part of the button.
Understand the context of when the form is displayed and what the user is most likely to do in that scenario, register or login. Let us say you display the login form with two fields and the user isn't already registered with your service, you provide a convenient link that says something like "Or Register" . Once the user clicks on the Register link you can gracefully animate the one extra field.
A very rough implementation of what happens when
- Sign Up is clicked in the Sign in View
- Sign In is clicked in the Sign Up View