Yes! I have seen what you would call a doublesided plate. If I could retrieve it, I know where there is a "plate" of crystals that looks like a rectanglar block about 2 feet long by 1 foot across by 5-6 inches thick, completely covered in points, all the way around it. It is about a 16 mile hike in here in the Rockies on the Montana/Idaho border. VERY rough country! If I could remove it and carry it, It would be in my home! You have to think, the crystals formed in a "pocket" or void, much like a huge "bubble" in the rock when it was molten. As it cooled, water got in and formed the crystals. The bubbles can really be almost any shape, and size you could think of. They can even have formed in "cracks" in the stone as it cooled. It's not too far fetched to think two vugs (or voids) could have formed really close together like two "bubbles" right next to each other, which could be one explanation for why your "plate" would have crystals on the front and back so to speak.