You can use them together but dont put anythimg in between them.
Your roots and plants will suffer some. It will impede their growth.Black on black gravel is a great combo mix.
It shouldnt look that obvious, especially with plants covering up most areas.
Go for it, i say. ;-)
I usually put the eco and or flourish on the bottom
then the regular gravel on top.The roots will seek out those great nutrients and grow longer for more beautiful plants.