How exactly are attacks distributed between your 2 weapons. Say I'm wielding FoA +5 and Scarlet Ninja-to for 5 attacks, how many times will each weapon attack? How does this work exactly? thanks...

I'm pretty sure you only ever have 1 attack with your off hand, so putting Scarlet Ninja-To or Belm in your off hand gives you an extra attack in the main hand.

In your case you'll hit 4 times with FoA and once with SNT.