Think about how you're making things happen.
Having HEALTH and SHIELD affecting your final health (that health seen by the player on the screen), you're able to make separated events to reach it.
For example, to represent the health on screen, use a math like HEALTH + SHIELD (those are variables, the the health with lowercase is a simple text on a text object in your screen).
So, if your player have 95% of HEALTH, and gain a powerup of +10 of SHIELD, your screen will show the 105% of health, simple using the math inside a text object:
everytick -> Text object: "Health: " & HEALTH + SHIELD
Plus, it can improve a lot your work by simple checking wich one you want decrease when hitting an enemy, for example, while physical hits can affect your SHIELD variable first, magical hits can affect your HEALTH variable directly.