How Do I Create Macro to Switch triggered by single keyboard button?

If I understand you correctly, you want a Macro to be used with iTunes, which will click a specific button the first time the macro is triggered, and then will click another button the second time it is triggered.

If this is correct, then you should be able to use the pattern used by this Macro:
MACRO: Toggle Start/Stop of Macro [Example]

Of course, instead of stopping (cancelling) the macro, you will want to change that to click on your second button.

Take a look at this macro, and let us know if you have any further questions.