The blue bar would count before the green bar, so there's no real reason to separate the two from right to left. You'd also be in rather dire straits if your health was so low that the shield was the only thing showing and you got hit hard enough for the shield to break + your health to deplete to 0. That's a very small, niche problem that wouldn't be somehow solved by being able to see your actual health. It could also be solved by making the shield chunk that's more than your health a different color - maybe a darker blue or a lighter blue, that way you still know how much health you have. You could also, y'know, look at the actual number.

IMO, it hangs off of the green part from right to left and moves with the HP bar. So if you're shielded @ 50% for 10% of your health, 10% of the right side of the green bit should be blue. That way it's fluid and adjusts with how much health you have without causing any problems.

It would be a nice QoL thing and I'm all for it. Probably won't get implemented or we'll have to rely on an add-on to make it happen, but still.