How does one pick the two weapons one wants to fast "X" switch between?
X is not fast switch - it is "switch to previous weapon".
If you want to switch between Assault Rifles and Shotguns (or whatever), you just need to make sure that those are the last two weapon types you've actually used.
The two weapons you switch between with X are always the last two weapons you selected from the weapon wheel. There's no way to override this as far as I know.
I learned more about weapon switching when I asked this question, which may be relevant to your interests:
How do I switch between more than 2 weapons?