Masking WKInterfaceLabel objects

I have a WKInterfaceLabel that spans two other objects (could be most anything, but generally solid color). (I'm using Interface Builder). One object is light and the other is dark. I'd like the part of the label on the light object to be black, and the part on the dark object to be white. See the example below.

Is there any way to "mask" out part of the label so that the objects behind show through? I could have two labels, one black and one white, but would need to mask out each half. I can't simply half cover each label to mask it out because that will mask out the other.