Derating a dimmer when using LED lamps?
This ceases to be a factor if you use dedicated LED luminaires which don't have sockets.   LEDs are so reliable, and they don't "burn out" like bulbs, that there is no reason to socket them and a lot of reasons not to - such as this.   

This also opens the door to using more robust dimming controls like 0-10V, PWM or smart dimming. (The system you are currently angling for is "the old way of dimming" that is made for incandescents, and LEDs struggle to work properly because it works the exact opposite way LEDs do.)

If you really want to use those fixtures, you can switch their sockets to something like GU24 which do not support incandescents. However GU24 screw-in LEDs are notably more expensive.  Candelabra sockets are readily available in LED at no cost disadvantage, but incandescent candelabra bulbs are commonly 40W and can be found as large as 60W.  

If you really, really, really want to support incandescents, then use 2 dimmers and call it a "feature". However, you have other problems. This with the dimmers consumes more than 50% of a 20A circuit's capacity, so it must be on a dedicated circuit that serves only hardwired loads and does not power any receptacles.  Like any continuous load, it must be derated 125% so this counts as 1500W toward hard circuit limits of 1800W (15A) or 2400W (20A).