Like Mike said, make sure you clean out the holes and use a good silicone or liquid nails and it will hold tight.
Lowes sells the Red Head adhesive for wedge/sleeve anchors if you are concerned with weight. It is a two part mix in the tip epoxy that you can get in standard single caulk gun style tubes. It is about $15 a tube but is mega-strong. Personally, I think that would be overkill on acrylic Gemini letters and going with off the shelf silicone would be best.