I had a Mandy for years in the 30 gallon seahorse tank. Dragonets, like Gobies, like to dig but if the tank is large, like yours, it can support a couple. Since they only dig in a limited area in the course of time they really do not disrupt the bed to the degree needed to really mess up the bed and impede its function.
The real problem is us. Some reefers just can't resist the urge to vacuum and clean the bed all the time. This creates constant flux in the bed and can ruin its functionality.