Still, I don't think you're blocking 100% of the UV, or your plants wouldn't be getting what they need, which includes the full spectrum of sunlight, right?
Think of it as the Aluminet shading out half the sunlight getting to the polycarb, then the polycarb filtering half of that. So they're still getting 25%, I think.
Edited to add: Welcome Bud, and what gorgeous Vandas you have!