I read from a post that dupont and drytreat both do not recommend against using two sealers. I got my info from this short thread
https://www.troublefreepool.com/threads/89034-Oklahoma-Flagstone-Coping-concern-and but will post what a forum member posted here.
"We have a new pool with OK flagstone coping in the Austin, TX area. I sealed the stone first with Dry-Treat 40SK, waited a month, and then went on top of that with StoneTech Enhancer Pro.
Dry-Treat is both a sealer and consolidator that penetrates into the stone and fills the microscopic pores, where this hardens the substrate, resulting in protection from water (e.g. staining, freeze/thaw, salt), cracking, chipping, etc. Dry-Treat claims to use smaller particles than StoneTech and other brands. Dry-Treat 40SK does not alter the color of the stone. After the sealer cures, the stone retains its original look. Within a week the stone returned to its original color.
Enhancer Pro is a semi-penetrating sealer that provides protection from water and oil, while also altering the color of the stone. This product will darken/deepen the colors, similar to what the stone may look like when wet. I would describe the effect as adding "drama" to your pool... The stone colors were pale and flat originally, but after applying this product the colors really pop, and you can see all sorts of subtleties in the stone that weren't visible before. Two coats are recommended for the greatest effect.
Both manufacturers recommend against using two different sealers. I went this direction only after allowing Dry-Treat to fully cure (directions state 3~4 weeks), with the goal of gaining the color benefit from Enhancer Pro. I figured it would be like using Enhancer Pro on granite by the time the Dry-Treat cured. I tested in a small area before going whole hog, and the results were promising. Looking at the coping now, having completed the full perimeter of the pool, nearby steps, and several stone benches, I'm very happy with the result and would do it again. I've used these products on Austin white limestone, Leuders limestone, and Oklahoma coping (buff peach).
I spoke to both manufactures before proceeding. Dry-Treat described their product as liquid glass. Microscopic silicon-type nanoparticles that are delivered using solvent to penetrate into the stone, with far better penetration than water-based products. StoneTech uses a similar technology, but they don't claim to have the same depth of penetration. Dry-Treat boasts 15+ year performance, while StoneTech claims 3~5 years (and some here reseal with StoneTech every 1~3 years).
StoneTech Salt Water Sealer and StoneTech BulletProof are two alternatives to Dry-Treat. I've purchased some of these for use in other areas of the backyard (kitchen, patio, etc.) and will compare their performance over time. The downside to Dry-Treat is the high cost. I used something like 7 or 8 gallons of Dry-Treat, and then followed up covering the same area with 2 gallons of Enhancer Pro. In testing, I found that Enhancer Pro provides water protection with just a very thin first coat, so the product goes a long way. I'm guessing this thin coat is what gives the limited lifespan, needing to be resealed in a few years. Dry-Treat directions required many coats on the stone, as I found the OK flagstone to absorb a lot of the product. I'm guessing this is where Dry-Treat shows its value as a consolidator, with more particles penetrating deeper into the stone (several millimeters).
If you have lots of time and aren't on a tight budget, Dry-Treat is a great product. If you want fast and low-cost, go with any of the StoneTech products. "
