16GB max is either 2x8GB or 4x4GB, both of these setup runs the ram in dual channel.
Already have a 8GB, then adding another 4GB making it to 12GB will be dual channel, but once you
add another 4GB to max out at 16GB, it becomes single channel.
Grab another 8GB and upgrade to 16GB.
If that board is actually that Pro, then maximum ram is 32GB not 16GB.