Do shadows stack? In reality, shadows can only get so dark. If you’re wanting a darker shadow, you could duplicate your text layer, set it to the color you want, then position it below the original, so that it’s roughly where you want the shadow to be. Then parent it to the original.
Finally, use a copy of the “shadow catcher” layer as a track matte for the shadow, so that it only appears on the layer. – As John says: fake it.