UI: Shields should add "Blue" over your Health Bar.

[TouchandFeel]

If it progressed from the right side, you could still potentially get overlap that would obfuscate the player's HP bar.

If it started from where the current HP total is you run into a number of problems since that is a variable point that will continue to change, also if the player is at full there is no additional space on the bar.

I'm not saying it couldn't be done, just not quite in the way the op layed out. Really there would need to essentially be another bar for shields that would either be extended from the HP bar or below it, but that then puts it in the same boat as having a visible to bar which I personally feel is way more necessary than a shield bar.
Would be nice though to have the option to have these things though.