In my testing, the different protection enchantments don't stack effects on armour. The most noticeable was using Protection 4 and Blast Protection 4. With just Protection 4 you take 4 hearts of damage (8 damage), with just Blast Protection 4 you take 2 hearts of damage (4 damage), but with both Protection 4 and Blast Protection 4 you still take 2 hearts of damage (4 damage).
At first, I thought it might be because I hit the armour's maximum protection percentage, so I added more TNT (to make the setup in the video below), and the numbers were still the same.
Cannot reproduce. I turned off natural regeneration, then I gave myself a stack of golden apples. I then gave myself three zombie heads: one with protection 5, one with blast protection 5, and one with both enchantments at level 5. I tested each zombie head by eating a golden apple before summoning a tnt inside myself. I only survived when I did this while wearing the zombie head with both enchantments.
You did take less damage with both enchantments, but it was not enough of a difference to be visible.