Is there a max width to the health bar?
From this video showing the (slightly outdated) maximum HP, the maximum was 2102 HP with 99 Vigor, a Life Ring +3, Ring of Favor +2, and being embered.
The HP bar at this value looked like this:
Based on the HP value of 2102, it is clear that the HP used to calculate the bonus from the Life Ring and Ring of Favor includes the bonus 30% from the player being embered.
However, since this video was uploaded, the Ring of Favor +3 was made available.
At 99 Vigor, you have 1400 base HP, so with ember and these rings, you would have:
1400 + 420 (Ember) + 182 (LR+3) + 109.2 (RoF+3) = 2111 HP
The bar will never become two bars, there is a cap on hp. With diminishing returns vit 99 will grant the player 1400 base hp.
Life ring +3 is a further 10%. Ring of favour +3 is 6%. Finally the ember is 30%.
The maximum health is 2111.
That is the max hp (and width).
Edit: Turns out I was wrong and multipliers stack, sorry for the misinformation.