Using Secondary HSL in Lumetri on a masked part of a clip
You do need a second copy of the Lumetri effect, but you don't need to duplicate the clip, whether or not you keep the extra copy.  Instead of your current workflow, try using Adjustment Layers:


Perform your primary color corrections as you normally would on a single lumetri instance.  By "primary color corrections," I mean white balance, exposure, overall adjustments, and anything that affects the whole frame equally.  No matter what color grading software you use, it's best practice to keep these "color primary" operations logically distinct from operations which require keys, masks, mattes, windows, etc.  One reason for this is that keeping primary adjustments logically distinct allows them to be baked into LUTs (a topic for another discussion).  But as you've discovered, it also helps keeps your work organized so that local adjustments can be accurately targeted.
Add an adjustment layer.  To do this, make sure your project panel is active by clicking in it, go the the File menu, and select New->Adjustment layer.
Drag the new adjustment layer from the project panel to your timeline and drop it on a layer that's above your footage.  In a sense, Adjustment layers are "empty" because they don't contain any video information, but in another sense, they're "transparent" and let you stack effects.  It's because of this transparency that you don't have to duplicate the underlying lumetri primary correction that you performed earlier.  The white balance, exposure adjustments, etc, "show through" to the adjustment layer.
Draw your mask on the adjustment layer, and perform your "color secondaries" there by adding a new instance of the Lumetri effect to the it.  There, you can brighten the faces as you see fit, you can keyframe the movement of the matte if you need to (aka "rotoscoping"), and your color primaries will remain intact.
In this way, you can add as many adjustment layers as you like, and isolate as many different areas of the image as necessary, each with its own unique color transformation.  You can even use mask blending modes to perform boolean operations to the masks, generating complex unions, intersections, cutouts, etc.  


There are some limitations and restrictions to Adobe's color workflow, which is why people turn to other software as their grades become more complex.  Tracking multiple masks is problematic, for instance.  However, for basic color work, Premiere is often a simple, quick, and effective solution.