Perhaps the easiest method and the method that would need the least maintenance is sprites. You could sprite 2 life bar backgrounds onto the screen, then the bars themselves would be sprites too, but positioned and sized to suit how much health is left.
One thing I like about this method is that you only need to update it when a character is hit, the other 99% of the time it's just being drawn automatically. For instance you might make a function to update the bars, just one for this example. Lets assume that your overall health bar length is 256, height is 16, and your health bar backdrop has a 32 pixel wide margin.
So to draw the sprites:
sprite 1,1,0,0
sprite 2,2,32,32
So thats sprite number, image number, X and Y
And to size sprite 2 to suit a health value, 0 to 100:
size sprite 2,health#*2.55,16
This will support alpha transparency, so no problem with using neat drop shadows or even making the health bar translucent.