It’s not about using both at the same time. Rather it’s that a smart Bulb should always have current running to it. It’s all right if there is occasionally a power outage, but if you are regularly using a switch of any type, either dumb or smart, to completely cut power to the bulb, the inrush current when power is restored can damage the radio over time, making your expensive smart bulbs that much more expensive as their life can be significantly shortened.
This is why the user’s guide for all the smart bulbs will tell you to leave them on power.
Fortunately, as I mentioned, there are a number of ways around this issue which will give you a natural switch on the wall while not damaging the bulb over time.
Again the FAQ will discuss the details: